Well, you said your tangs are small, so they're ok for now, but they'll definately need a bigger tank in the near future. If you have the $$$ now, i'd say go for it, and get a larger tank... Mind you, if you don't want to have to upgrade again in the future, you should at least go with a 90 gallon or bigger, 125 being ideal!
If you're not attached to the tangs yet, see if you can trade them in at your LFS.
On a tank that size, i'd say a few cleaner shrimp (they like to be in groups). A few firefish (they like to be in groups too). Some sort of goby, i always recomend the Yellow Watchman Goby, or maybe some type of jawfish (these don't like to be in groups). These are all really hardy and easy to care for, and loads of fun to watch! I'd definately have a few firefish if my tank weren't already fully stocked...
Jenn