The regal will get too big within a year , maybe two at most - they need swimming room. Clowns are excellent for smaller tanks - how about adding a nice omnivorous blenny like a midas or bicolor and then perhaps a goby. You could do 3 or 4 fish if they stay relatively small - you can also control waste production by scaling back on feedings though this can be hard to do - and I hate people that starve their fishes to maintain water quality.

A couple ideas for stocking:


2 x false percula clowns
1x midas blenny
2x neon gobies


or maybe

2x false percula clowns
1x pygmy angel
1x watchmen goby

or how about this?

2x false perculas
2x yellow head jawfish (you need to make sure rock is sitting on glass and they have a mixed substrate to build)

or even

2x false percs
2x bangaii cardinals
1x misc goby or blenny

Lots to work with but any tangs, large wrasses, most angels would be out IMO. You might do ok with the clowns and a single dwarf angel also - though personally I'd give them a slightly larger more mature tank.