Well, I keep mine with a senegal poly, a uaru and chocolate cichlid, and some convicts. The cons aren't permanent, and it they cause any trouble I can move them out. The chocolate and uaru are very laid-back cichlids so they seem to work just fine, but I definitely would avoid more aggressive cichlids and more aggressive-feeding cichlids (like oscars). I think dats would work better than big cichlids. Also gars tend to equate silver fish to food so it's best to avoid them, though I do hear of gars kept with pacu and arowana so it can work.
As for the knives...big ones like clown knives are probably a bad idea. I'm considering trying a black ghost knife so if anyone has any experience with that combo I'd love to hear about it. Cats seem to be a poor choice as they seem to be either too big/greedy/aggressive for the gars or too small for them. I was considering a lima shovelnose as the sizes seemed comparable but not sure if that's been done successfully by anyone.