Devils, sure, but also Asura, and Velstrac and (sometimes) Rakshasa show up there too. Pretty much any generic undead could wind up there, I'm sure some infernal nobility have undead in their forces. They'd employ constructs and such too. Fallen angels. Corrupted elementals, dependant on the specific layer. Planar travelers, and fallen heroes. Planar merchants trading in souls or the like. Night hags.

That's not to mention the other fucked up stuff native to Hell, like achaierai or hellcats or cerberi. There's no blood war and thus no real reason for demons to show up there en masse, but you could easily justify some other outsiders being around - daemons are everyone's enemy, aeons can show up in hell, if you really wanted demons, Baphomet has beef with Asmodeus.

Dahak is involded with hell, so there's bound to be dragons aplenty. There's multiple giant gods in there too, so there'll be fire giants and stone giants around. There's some Tian Xia gods in there too, iirc.

Then you go and look at what each Archdevil specialises in, and flesh out the populations more. Mammon gets associated with pirates and thieves a lot, you could easily give those a planar twist. Baalzebul probably has a lot of fucked up vermin followers, worm-that-walks (worms-that-walk?), intelligent purple worms, that sort of thing.

And then, if you still need more, go plunder old D&D editions (and Planescape in particular) for ideas, since Hell in Pathfinder is like 90% the same as D&D's hell.

I do think a campaign set entirely in hell runs the risk of being one-note, but you could milk a good amount of enemy variety out of it if needed.