I have about 30 guppies of varying sizes in a 29G planted tank. I also have 3 cory cats in there as well. Due to an unforeseen event I had to relocate a peacock eel in this tank...It would nip the finage of my wifes fancy goldfish and go for rides! I have seen this eel hunt the baby guppies, which are a perfect size for him! I suplement his diet every week with a ball of frozen bloodworms, but he is in no way starving.
Eels are good because they aren't the the best hunters, but will probably do well to keep the population controlled. Bichir's are to aggressive, they would go for the bigger fish that would be the baby makers-soon there would be nothing breeding to maintain food levels.