1) Get a shoal of about 5 yo yo loaches
2) A smaller bichir like a senegal or delhezi
3) Leopard Ctenopoma (I'd make sure that this grows up with the knife together to avoid trouble)
4) Cory Catfish
5) Australian Rainbow Fish
6) Congo Tetras
The list goes on. It really depends on the size of your tank.
Pikes can be pretty aggressive. What kind of Pike is it? Depending on what type, they can get pretty big and become very predatory. I would go with Congo Tetras, but again, depends on the Pike. You can go with a shoal of Silver Dollars.
Hmmm...I think they average 6" (don't know much about Pikes). You can probably get away with Congo Tetras since they are fairly larger Tetras, but maybe stick to the SDs to be on the safe side.
African knifefish are not aggressive fish. They tend to be peaceful with fish too large to eat. You might try a leopard ctenopoma or a smaller bichir species that maxes out at 12 inches. I would not recommend anything more aggressive than these.