so, I've checked numerous sites for my specific bichir,
Polypterus senegalus, and they for the most part have all said that in captivity that they rarely get to 12" long and that a 55 long gallon would be fine. as far as the clown knife, yes i am aware it will get stupendously large, but i also know in my area they are quite popular so it shouldn't be a problem getting rid of him/her if it gets too big (i also hear you can eat them and they are quite tasty...).
my big goal fish-wise is to keep the bichir for sure, another african butterfly, keep my poli for cleaning, and if not my two silver dollars a couple of pacu (that are supposed to get along alright with the bichir)... i found quite the extensive forum specifically for bichir here:
http://www.aquahobby.com/gallery/e_senegalus.php
I am quite serious about having fish, it's what I love, but unfortunately a bigger tank than my 55 will not be happening, further advice?