Yes, you can keep just one blue/green reef chromis, but they just don't look all that nice kept singly - there are better options for additional fish for your aquarium. I would also skip the mandarin/ dragonette, unless you are able to obtain a captive-bred individual or have experience training them to target feed - they tend to starve with typical feeding regimens in a reef tank.
A single (or confirmed pair) of firefish would probably work, although grammas and tailspot blennies can occasionally harass timid fish in smaller aquariums once established - yours might be big enough though (and hopefully covered, as they tend to jump).
As for (relatively) peaceful damselfish, there are a few that are considered much less problematic - take a look at Allen's, azure, neon, Roland's, Talbot's, and Tracey's. There are a few others, such as Starck's and canary, that are much more expensive and relatively rare, but are worth it.
You could also consider a shrimp goby, a smaller fairy/ flasher wrasse (if you have a cover, which you should regardless to be honest), a smaller hawkfish (Falco's or flame), or one of the smaller/ relatively peaceful dottybacks such as an elongate, sunrise, or Springer's (might not work with the gramma though).
You can also keep more than one royal gramma, but ideally they should all be introduced at the same time. If you have an introduction box (and your current gramma is not very large and mature yet), you could possibly introduce 2-3 additional grammas to the tank, but that might be pushing it in terms of stocking levels.
Your best bet might be a firefish, 1-2 damsels, and a shrimp goby or small hawkfish, added all at once (after proper QT of course) to ensure any aggression is not focused on just one fish.
