Again check out this link:
Any of these fish would be compatible with your setup and your clowns(1-2 maybe 3 tops). This is assuming you get rid of the YT and scooter. Not to be too harsh, but frankly if you do not get rid of them I will be suprised if they are still alive 3 months from now.
As for specific selections some of my favorites in addition to the percs which you already have include:
green clown goby:
yellow watchman goby:
You could probaly even get away with 1 dwarf angel such as a coral beauty:
If you go with an angel though no other fish beside the percs. Really it comes down to personal preference on what you like. Just do not over do it or you and your fish will pay the price. I speak from experience as somone who has sent my share of fish to the great reef in the sky.
As for other inverts to look at:
10-15 more blue or red leg hermits
10-15 snails variety is fine avoid nasarious unless you have a sand substrate.
Your tank would look pretty full with these. The key to this hobby is take it slow. Read and learn as much as you can before making an addition. The worst think you can do is thry to fill up your tank right away because you will be headed for a crash. A new set-up really is not considered mature until it is 6-12 months old.