Having kept a bichir or two in my day....I can shed some light on some of your questions:
1) The Senegal Bichir (Polypterus senegalus) is often available at PetsMart. They sell little 2 inch babies as 'Dinosaur Eels' for $4.99 each. If your local aquarium stores don't get them in often then you can expect to pay a premium price when they are available. I've seen sub-adult (6 inch) Senegals go for $20 each.
2) The absolute minimum tank I would suggest for a single (or maybe 2) adult-sized P. senegalus or any of the other 'smaller' bichirs would be a 30 gallon long. A 40 gallon 'breeder' is just about ideal for a small group (around 4) of the 'smaller' bichirs like P. senegalus.
3) Growth rate depends on a couple of factors. One is the amount and quality of food you feed them. A varied diet and a couple of feedings a day will produce rapid growth in young bichirs (as much as 2 inches per month is possible). Genetics also plays a large part.....just as in humans, some individual bichirs will grow at a faster rate than others that appear to be almost identical. The age of the bichir is also important. Young bichirs that are well-fed can exhibit dramatic growth. The closer a bichir gets to its adult size....the more slowly it grows.
4) Any 'meaty' foods will be acceptable. Of course, you have to tailor the size of the food for the size of the bichir. My smaller bichirs get frozen bloodworms, frozen pieces of shrimp, pieces of frozen beef heart, earthworms (chopped to appropriate size), and Hikari Sinking Carnivore Pellets (broken in half). My medium-sized to large-sized bichirs get the same stuff (omitting the bloodworms) but in larger portions. I also add frozen silversides to their menu.
5) Bichirs are generally peaceful but predatory. Anything small enough to fit into their mouth will eventually finds its way there. Occasionally you will get a bichir that is aggressive...but generally only to other bichirs.
6) Maximum size depends on the species of bichir. Polypterus senegalus, Polypterus retropinnis and the subspecies of Polypterus palmas generally average out at about 12 - 15 inches in captivity. The larger bichirs (like Polypterus endlicheri endlicheri, Polypterus endlicheri congicus, Polypterus bichir bichir, Polypterus bichir lapradei, and even Polypterus ornatipinnis) can reach lengths over 2 feet with P. endlicheri congicus reaching 3 feet or slightly more.
7) See the answer to Question 3.
8) In general, bichirs get along well with other bichirs. See the answer to Question 5 for a disclaimer, however.
9) I can't say that I've seen bichirs 'play' with other bichirs but they often do 'hang out' together. They are not, by nature, schooling fish.
10) Bichirs can live with most fish that they can't eat. Personally, I avoid keeping bichirs with a) any fish that could eat or damage them, like piranhas, wolf fish, large catfish, larger bichirs, puffers,etc.) b) any fish that is especially aggressive or territorial like large cichlids c) 'pleco' type catfish that occasionally develop a nasty liking for bichir slime and will rasp on the bichir's scales until the Polypterid develops nasty skin ailments which can be fatal.
Hope this helps a bit,
-Joe
