a pleco might be ok....but if he gets into there territory, he might be attacked....you just have to be vigilant......usually it is recommended a pair of kribs to have a 20 gallon all by themselves...plus maybe some fast dither fish...
Pleco should be fine. Don't know about the rasboras. When my kribs spawn in my 29gallon (same length as your 20Long) they basically take over the left half, and everyone else hangs in the right. Just make sure the most attractive "cave" is on one side of the tank so they don't claim too much territory. I have some cories and white clouds in with them and the cories share the same tank level, the kribs just chase them off and go back to the cave. I've not had any serious injury during a spawn. Probably depends on the individual personality of the fish though.
As with any fish they all seem to have personalities of their own. As was advised above just be vigilant.
My pair were sharing a 30g long with a large angelfish and they harassed the angel until I took him out. they are masters of the whole tank now