I don't really know whether to write
Welcome to the forum!
or not. So, since you're a long time reader, I won't. ;D
Anyway, the answer is basically yes. I would recommend a script called from a trigger rather than trying to do it all in the trigger, although if this is for MP then scripts are not always the best solution.
It's not possible to detect what is in an ammo crate, so the script will already "know" what is in the crate (because you put it there.) I haven't the time or the skill to write the script now but the gist of it would be that you take each loon in turn, generate a random number using the random command, and give him a weapon based thereupon. For example, if the random number is between 0 and 1 he gets an RPG, if it's >1 and <=2 he gets a PK, etc..