The ones you picked out are not large and should be just fine. The firefish and chromis will be the active middle water fish while the clowns will most liekly find a corner soemwhere and hang out there. At least thats been my experience with clowns. lol
I happen to have a firefish and really like him, he is always out and about. The chromis are a good choice too as they are hardy, cheapo and always active as well.
I wouldnt go with anymore fish unless you consider bottom dwellers like gobies as you will have a good amount of fish already. If it were me I would get a bottom dweller too, shrimp gobies can form relationships with pistol shrimp, it's awesome to see. The goby will bring food to the shrimp and they take care of each other. They dont swim around much but it is one of those things in nature that is great to watch. Just more options for you. 38g isn't a huge tank but it can house a good amount of livestock if you think it all out well.
Good luck! let us know how it turns out!