Depends... How big is your tank?
A lot of catfish grow fairly large and are unsuitable to be kept in small tanks or with small tankmates (as many species of catfish eat pretty much anything that can fit into their mouth). I have no experience with eclipse catfish/sun catfish, but if you want good tankmates for bettas, a shoal of cory cats is always a good option.
EDIT: In regards to plecos... Most common varieties of pleco get far too big to be kept with bettas. I have a pleco that is 10 or 11 inches now. Even the smaller varieties like bristlenoses produce a lot of waste (meaning you would have to double filter and perform water changes very frequently) and should ideally be kept in at least a 30 gallon.
Last edited by Fenghuang; 07-06-2013 at 07:46 PM.