Most Senegal and Delhezi bichirs are captive bred and don't typically grow to the size of their wild caught counterparts. I had a delhezi for about 3 years before i traded him for another bichir and he only grew about 2 inches in that time (from 6 to about 8 inches.) If your tank is 18 inches wide I think either one of those species would work as they are not likely to grow more then 12 inches and that may take years.
Other bichir species grow bigger and faster, but are typically more expensive since they are less well bred in captivity.
I have not kept the other fish you are asking about, but i do keep a black ghost knife with my bichirs and they work fine together so i would assume the brown knife would be ok as well.
Bichirs work well with most other fish as long as the other fish is too big to get eaten by them and not big enough it can eat your bichirs. The other main consideration with bichirs is that they are bottom feeders and they react to smell more then sight when it comes to food. Whatever you keep them with, you need to make sure the other fish will not out-compete them for food.
I feed my bichirs Hikari sinking carnivore pellets as a staple. They get treats of earthworms, black worms, ghost shrimp or tilapia on occasion. Black worms are their favorite and fun to watch them slurp up like speghetti. When i feed them sinking pellets i have to throw in some floating pellets to distract my gourami i keep with them or he would eat ALL of their food.
If that is indeed the case then imagine I will end up with the delhezi. When I first discovered bichirs I really wanted an albino senegal but after looking at the delhezi..I much prefer it.
Hopefully can make sure it's not out-competed by placing the food close to it?
I thought I read somewhere..of course doesn't mean it's true..that hikari contained something in their food that caused some fish to become more aggressive than they'd usually be.
This was on a cichlid thread though so probably doesn't apply to these pellets.
Thank you
D'aww pikachu :3