What are your recommendations for enemy placement?

Monster - or thing-placement - is something that is kinda intuitive for some mappers, while others hate to do it. Personally, I always look at it from the perspective of deliberately creating problems which can be solved with the resources players are/will be given.

Depending on your goals, monster placement can be a very simple matter, or a rather "complex" one.

If you want to just populate an area, so players have something to shoot, then that's not something that requires a whole lot of thought in my opinion. When you're looking at incidental combat, it's usually not particularly demanding (relative to the rest of the map's difficulty), so anything players can deal with in a reasonably short amount of time, ideally without dull and repetitive gameplay loops (such as SSG camping from the same corner for like 20 seconds or something like that) is most likely going to be okay.

When you're looking at bigger rooms, which have some more stuff in them, then you should probably look into how you can make players actually get into the room instead of encouraging a "campdown". When you are looking to make people move their lazy asses, then staying outside of that room needs to be unattractive enough so that people want to move forward, in which case what's in the room matters a bit less than what's behind the player. Alternatively, you can lock people in, and then make these monsters show up once the doors are locked proper, which will force players to fight their way out of that room.

When it comes to staging fights in a room, you need to consider how high the threat level of that room is, and how sustained the threat is. What matters as well is how likely things are to start infights... Here's where it's down to actual thing placement, rather than conceptualizing a scenario. One of the best ways to end up with a fight that feels "dynamic" is to give players no "uncontested area" from where they can "work", by means of using monsters that can "control" larger areas (chaingunners, revenants, viles, mancs) alongside monsters that immediately threaten the player's "real estate" and push them around a little bit.

Another thing to consider when "orchestrating fights" is how they develop, which depends on many different factors such as geometry, infight probability, how much cover there is, etc, but one thing that is usually worth avoiding is an "insta-circlestrafe" where players merely hold down the strafe key and press fire. It's a similarly dull gameplay experience as "cornercamping" is. So, when you're looking to build something that lasts a bit longer than a few seconds, try to make circlestrafing at least an unattractive option, rather than an obvious solution.

If you want to read somebody else's opinions, there are several "guides/essays" available online, such as this one:

Keep in mind that this is all a matter of opinion and personal preference for the most part, so neither everything I said, nor everything that guide will say needs to be followed to the letter, especially not if it conflicts with your own ideas and ideals. If you really wanna "learn" monster placement, release maps, and have people give feedback.

It completely differs based on monster type.

Monster type tells you which weapons you need to make available to the player to defeat them. This requires familiarity with the game. 

The environment can make monsters tougher or easier, depending on how you want the encounter to go.

Whether you have enough health and armor makes the map more tense.

The sure way to know a map is playable is to test it by playing the wad file. If you can defeat every single monster on the map (if you have enough ammo to do this), you're doing good as far as monsters go. Source: I've only designed one map (it's in this site's queue to be uploaded) but I've been playing first person shooters for a long time.

Edit: also, only former humans will give ammo, so that's something to keep in mind.

You know what? In my opinion, just play the map. I place enemies where I think they fit, then play, and I am too low on ammo, or think "this part is boring", or "too many same enemies in this area", I change it and play it again. Until I think it's right. This includes adding level geometry to make it more fun to navigate or give you cover.