This is perhaps a little simplistic; I myself would think more to the judgment used by TeamTNT, TiC & associates, etc. in the early classical megawads as a more general judgment. Indeed most monsters have been introduced earlier than in DOOM2.WAD in "normal difficulty" megawads without seeming wrongly early. By "normal difficulty" I mean the more old-school releases such as Icarus, the Memento Moris, Requiem, TNT Evilution which were meant to be played by a more typical player base (as opposed to such wads as Eternal Doom, Plutonia, and of course anything based on the Hell Revealed style, which are clearly designed for a more advanced player and therefore do not really enter into this discussion).
I think, as a rule of thumb, that monsters should not get introduced earlier or on lower difficulty settings in Freedoom than in Doom 2.
Let's go monster by monster:
(Bullet Type Guys)
TROOPER - Not much to say, typical MAP01 fodder.
SS WOLFIE - Mostly irrelevant to the discussion due to primarily factoring in as an 'easter egg' type enemy. Few wads use these guys in any sort of "normal" capacity, anyway.
SHOTGUN GUY - Although DOOM2.WAD didn't include any sergeants until MAP02, many wads have these on MAP01 (indeed, MM2 starts you off facing at least two in the very first room on UV). Furthermore, in the original DOOM, these appeared on E1M1 and could easily be the first or second monster the player even meets during their entire DOOM career! No reason to restrict from any map. :)
CHAINGUN GUY - These can be dangerous, but that is highly situational and they are not out of place even as early as MAP01 (e.g., MM2 - of course, keep in mind that you are immediately in close quarters with the first ones you meet and have obtained a shotgun before then, so they're not too tough.)
IMP - Typical MAP01 fodder, again.
CACODEMON - This is held back to MAP05 in DOOM2.WAD, but appears as soon as MAP03 in MM2. To be honest, not too mean to a new player at all, unless the player is in tight quarters or is still only able to use the peashooter (and yet the latter situation is inflicted on a player at start-of-episode even in an id map, i.e. E3M1). Even a chainsaw encountered before encountering a Caco makes them reasonable even for an early map (and, indeed, they appeared in the first map of every episode of the first Doom except for E1, and likely the only reason one wasn't snuck in even there on UV was probably because of marketing strategy, i.e., the decision not to feature certain monsters in the shareware version).
HELL KNIGHT - Like the Caco, the important thing here is to let the player adequate dodging space. Like the Caco, id delayed the Knight to MAP05, but MM2, for instance, made good use of it in MAP03.
BARON OF HELL - Like the Caco ... getting a pattern here? :) This is why I classify these four monsters together, because there really isn't very much difference between them except for damage and hit points. Being a little more durable (though no more deadly) than the Knight, perhaps it's better to let the Knight be a more gentle preview. As for when it's right to introduce the Baron, both MM2 and Requiem, to name two, had at least one on MAP05 on UV. (For the sake of the Freedoom level - perhaps, especially given the slime, it is a little early to have two in such a small area. A Knight on skill 3 and just one Baron on skill 4 sounds like a good change.)
(Strictly In Your Face)
DEMON - A mob this basic can easily be used on MAP01 without complaint, no problem. (And MM2, for one, does just that.)
SPECTRE - Complaints about spectres largely are a matter of terrain and lighting, not so much the monster itself, being that it's just a blurry Demon. Nothing wrong with MAP01 here either.
LOST SOUL - These are even weaker than the Demon and Spectre. Pretty sure I've seen them on MAP01 in some or other PWAD of the difficulty type I've noticed above but I'm not quite sure. I do know you'll find at least a couple on Requiem MAP02 though (due to the solitary Pain Elemental on that map).
(More Special Cases)
ARACHNOTRON - These have a tendency to be featured first on MAP07. However, they are a rather easy enemy as long as there is space to dodge (especially if they're only coming from one direction which is a more "normal" situation). It's hard to say what the usual judgment should be on these primarily because, like their bigger counterpart, they are a rather large radius monster that's hard to place effectively in many situations.
MANCUBUS - A fairly advanced enemy owing to its large, damaging fireballs that can be a pain to dodge, much more than the single shots of the Imp-class enemies or the Arachnotron. Although it's probably first introduced on MAP07 in DOOM2.WAD due to an apparent desire by id to surprise a new player with two all-new monsters, it's also an enemy that probably should be introduced sparingly in a normal difficulty wad like Freedoom (even Requiem, probably the hardest of the megawads I'm considering here, doesn't introduce them till MAP07. MM2 introduces a couple on MAP06, TNT Evilution has them all the way back to MAP03, but only sparingly per map).
REVENANT - The key here is COVER. With it, pretty much the same sort of ideals that apply to the Hell Knight or Caco apply. Without it, it's a lot meaner and should perhaps be introduced correspondingly later.
PAIN ELEMENTAL - Like Chaingunners, it's heavily a matter of numbers and location. Far away and in groups suck for the player, while a solo pain elemental up-close does not make for an especially serious threat (e.g., Requiem, which introduces one as early as MAP02).
ARCH-VILE - Ow. An early archvile is usually an indication of an advanced difficulty wad. DOOM II didn't sic one on the player till MAP11 for a reason. MM2 waited even later (MAP12), and Requiem further still (MAP14). The original Memento Mori did introduce them sooner than most (MAP07 on skills 3+, though MAP12 on skill 1-2). I'm not sure off the top of my head where Freedoom has its first one, but this is definitely usually the last enemy (other than an Icon Boss) to be encountered by the player after starting a wad. Again, cover is vital to avoid player frustration.
CYBERDEMON - Some players retain an eternal fear of these guys. Their size, also, generally excludes them from a lot of maps (although they do not necessarily need to be able to move or even threaten an especially large area to be part of a good challenge. Viz. MM1 MAP30). Not something the beginner wants to bump into too soon, and, indeed, even classic PWADs shy away from introducing one earlier than DOOM II's MAP08 (most of the type I've been referring to seem to wait till even later.
SPIDERDEMON - A mob that's hard to place the real difficulty balance of; the true problem here being that its enormous radius means that very many levels simply cannot feature one for lack of a suitable location. Between this and the fact that the player pretty much needs significant firepower due to its being a hitscan enemy and thus essentially impossible to dodge, Spiderdemon levels are rare and cannot really be qualified by a level number. (While Cybs are not too uncommon even in DOOM2.WAD, especially if one plays co-op, there are just three levels in DOOM II that had a Spiderdemon and they are all widely separated).
ICON OF SIN - Having one of these other than MAP30 (or in the case of using the Romero head for technical or special effects purposes) is uncommon, but a few maps do have this. Mostly this is a thing for late maps in a WAD though (of the 'classical' megawads being discussed, Requiem was the only one to feature a pre-MAP30 Icon, as the end of MAP23), the player really needs time to get used to the game first!