Question: -Is there any way to pick up the lowest level creature in a group (so that you can get them to train)?
-Workshops automatically produce at least one of each type of trap/door, but i dont want workshop space to be taken up by useless wooden doors. Is there any way to get the workshop to NOT build certain traps/doors?
-Is there any way to exclude certain creatues from my dungeon other than to kick them out of via the portal manually (e.g. beetles, they keep showing up, but i dont want them)
1 - Hold Shift while mouse clicking the creature icon in the creature panel. Not the inactive/active creature numbers, the little picture. Holding the Ctrl key while clicking picks up the highest level creatures.
2 - No, not without editing the level text file to remove them from the script (see below). Workshops create whatever is available to build in a level in a specific order, one at a time, than repeats the cycle. So if the level script says it's available to build, they're going to build it. Basically, you just build really large workshops, and/or use a long hallway to plonk excess doors/traps in and sell them (don't have to wait for imps to place them to sell them).
3 - No, not while in-game. If you feel like changing the level text files that are in the levels directory, you can REM out the relevant CREATURE_AVAILABLE line. This makes them completely unavailable through portals, period. I wouldn't generally recommend doing this tho, unless you're familiar with the game/scripting and know what you're doing (scripting isn't very hard, tho). If you decide to try it, make copies of the text files in case you won't remember the changes you made and want to go back to the originals.
If you have specific creatures you want, they have certain things that attract them, so make those rooms as large as possible. eg, dragons like treasure rooms and big lairs, so build giant treasure rooms (or lots of smaller ones) and huge lairs and you're likely to see more dragons.