You can keep 2 firefish in a 10g as long as they're male and female (not always easy to tell apart,, look for two hanging around together at your LFS). Firefish are good jumpers so a covered tank is definitely recommended.
I have the following in my 75:
Pair pink skunk clowns (spawning)
Pair Firefish (spawning)
Pair yellow Clown Gobies (spawning,,, waiting for tonight's hatch right now)
Molly Miller Blenny juvenile (temporary resident I think,,, won't stop nipping at corals)
Pair Oscellaris clowns - Female is a Black & White,, male is standard orange,, acting like they're going to spawn soon.
Yes, I like to see happy, spawning fish. I wouldn't be pleased if someone locked me in a cage with an armadillo, a giraffe, and a bunch of other stuff I couldn't hump, so I try not to force that situation on my fish.
They are all fairly small fish,,, they all have their territories and there is very little aggression. With all the lurve going on it's a very interesting tank. I don't recommend mixing clown species,, I've been lucky and will probably remove the oscellaris to another tank when they start spawning.
Chromis and Clowns aside, I'd stay away from damsels as the most common/cheap types can get cranky as they get older. That's just my personal experience,, I'm sure there are others who have really nice old damsels.