I like perculas, but if I ever have them again, they will be the only fish in the tank. We had a pair, and they were buggers. They look so cute and happy, until you see them pummeling another fish to death, dragging it around so it would get sucked into the overflow box...Dreadful. So, if you want a pair, you can have those two. They will pair up, and get along fine, if you start with them small. You could maybe add a small goby, or chalk basslet, but the basslet would rather be in a small group. Any other member of the damsel family (other clowns, damsles, or chromis) would be out.
Some options to look at: Chalk basslets, tobacco basslets. Both stay fairly small, and will be happy in your tank. You can look into the pygmy angels--bicolors, potters, rusty, and coral beauty angels (a single specimen only) would do fine, and could go with the basslets or a pair of perculas. There's a long list of inverts you can have--shrimp, crabs, stars, etc.
Once the tank is established and has a decent growth of algae, a lawn mower blenny is a great addition. They stay small, and are quite comical--you'd never suspect their mouth is really THAT big!
On the odd ball side...A 29 could be setup easily for an octopus. They are really neat creatures, but have a short life span. Very easy to maintain in a stable tank (at least 6 months old). Or, check out some of the small anglers out there. Some are very pretty-brightly colored, goofy little guys who seem to walk around, looking for things to suck down. Either of these options would limit you in some way--octopus eat anything they can subdue (hermit crabs are a great snack, mantis shrimp are delicacy, etc).