[Fruitbat]

Glad to be of some help!

1) Three Ctenopoma acutirostre will be OK in a 20 gallon tank for the vast majority of their lives. There are reports that these fish can get up to 10 inches long but I've NEVER seen one even approach that size. They will eventually get to the 6 inch range though, but it will take a while.
2) I keep my Ct. acutirostre with some similarly sized Bush Fish of other species, some bichirs, and some African Butterfly Fish. They all get along splendidly. Most Bush Fish aren't aggressive (though they are predatory) and they don't do well with fish that are aggressive because they won't defend themselves.
3) It is OK to feed them baby mollies as long as you sure the little ones aren't carrying any kind of disease or parasites. I will caution you, however, not to make live fish a major part of their diet or you might find that they'll begin to refuse other types of food.
4) I've never noticed any real difference in aggressiveness between males and females, though it is REALLY difficult to tell the genders apart without taking the fish out of the water and examining them with a magnifying glass.

-Joe