I would say at least 55gal, the bigger the better. The smallest bichirs still grow to about 10". They like to eat bloodworms and carnivore pellets. Whenever we have meat or fish, I also give them tidbits of those. Like steak, salmon, or chicken. I pre-chew it to get all the sauce or breading off first. :P I don't know about mixing different species, but I would -think- that it would be ok. They seem to like having company. I recommend polypterus senegalus for a beginner. They seem to be very intelligent and will swim up to the glass while you're in front of the aquarium
Just a quick few questions: what is the minimum size tank for a birchir? If I am not able to get feeders, what should I feed? Can different speacies be mixed? Which one do you recommend for a beginner?