Personally, if I were you I wouldn't add anything else. Pleco's are known to suck the slime coat right off of an angelfish. Also, angelfish grow so quickly in size, and can mature by age 5 months the earliest. It really isn't worth getting anything else which will end up dead anyway. A pair of angels need a 20 gallon tank all to themselves. Even if they don't kill the other fish, all the other fish will be miserable! They will ALL hover in one little corner of the tank to avoid being killed!