Another issue is... is it possible to dynamically spawn monsters real time via impulse? (like half life's impulse 76)

I tried adding the monster's model precache to void () worldspawn, void () StartFrame , but i keep getting "host error pf precache can only be done in spawn functions?" 

StartFrame runs every frame! If you put a precache in there, you will hit the error for a late precache just about as fast as it's possible to hit it. Stick to worldspawn and you'll have a better time.

But as long as you time the precaches correctly (always on frame 0, and never afterwards), you can spawn monsters dynamically, in essentially the same way as creating a rocket or grenade. One small catch is that you can't update the total_monsters count after the first frame - so your monster kills can go over 100% unless you ensure that dynamically spawned monsters don't count for the kill count.

Another tricky wrinkle to deal with is where to place the monster so that it isn't stuck. Usually that's the responsibility of the mapper! One thing's for sure, you'll have to come up a compromise, and here's the proof: imagine that we make a map that's just a box 8 units larger than the player. It's physically impossible to successfully spawn a monster into that map. 

One other thing to warn you - don't ever randomise your precaches! Imagine you had some plan to randomly select one monster which your impulse spawns for the duration of the map, and you make the selection in frame 0 then precache just the assets for the selected monster. This exposes you to an interesting bug!

The bug arises when you save and load the game. In particular when you load the game, the engine:
1) Runs the first few frames of the map as if you had started from scratch
2) Reads the entity states from the save file and shuffles everything around to match it
The point of 1) is mainly to precache everything again. But second time round you might roll a different number on the random monster table and precache different models. Things may start showing up with the wrong models, or if you are less lucky the whole thing just crashes.

The only way to avoid this problem is to make the precache sequence entirely deterministic. If you're married to the random spawnable monster idea, you might have to precache the models for all the options, even if only one is ever used at a time.

And there are other more subtle ways to hit this error. Imagine you have an entity which you only allow a 50% chance of manifesting, randomised on each map load. Even if you have so many of this entity that one spawns every time, random variation might change the order in which models are first precached, as the first manifestation may occur before or after other fixed entities. Changing the order of precache messes up which models appear after a save is reloaded.