What type of clown is it?
Since you have had the clown for so long as a single, it is going to be hard to get it to pair. You'd probably be better off trading it in for a younger pair, but if you're like me, it's hard to get rid of a fish you've had for so long. If it's a Maroon, I'd say absolutely don't risk it. If it's an Ocellaris, your chances are much better. There will be aggression in the beginning, you just have to observe them, the new male could take a beating. (Your clown is a female).
The stocking options are really endless, but this is what I had in my 40G Breeder:
2 Ocellaris Clownfish
1 Lawnmower Blenny
1 Coral Beauty dwarf Angelfish
1 Spotted Mandarin (baby, ate frozen, knew I was upgrading and would be able to move him to a bigger setup)
1 Goby
1 other fish that I can not remember right now it was a small little guy
2 Skunk Cleaner Shrimp
10 or so Nassarius snails
20 Trochus snails
1 Emerald Crab
a handful of blue legged hermits (I don't recommend hermits, they kill snails for sport)
1 CoCo Worm
2 Hawaiin Feather dusters (Coral Beauty ended up eating them)
1 Rose Bubble Tip Anemone with Porcelain Anemone Crab