As noted by the Big Lebowski above the 13 gallon marker is important. What is more important than that number is what you are actually left with after you take into account power heads, heaters, rocks, and substrate. Yes, it has a false wall so heater and filter will be out but you may have a power head in there. Point Lebowski was making is that you really won't have that much water volume.
Ok - so moving past the obvious you didn't mention what sort of corals. While I only mentioned equipment above we also have to factor this in because they will grow. Frag today. Tomorrow a colony or a bush. They grow tall, wide, bushy, bubbly, and branches. Think of the song by Rush titled Trees. Oaks vs maples.
Moving on have you envisioned a reef type you want to replicate? Crest? Lagoon? Mixed. Specific soft, LPS, or SPS? Fish may prefer X of Y regardless of us hobbyist treating the display like a junk drawer and throwing everything in

Teasing of course but say SPS is what you want and you add a clown goby. Cute as they are the little rascals hang out in SPS which may irritate it or you.
You do have options. Small possum wrasse is nice. Calm. Chalk Bass. Royal Gramma. Gobies. Starry (biota), Links (biota), blue cleaner (biota). Pistol shrimp / goby pair. Pipe fish (not if you get a wrasse due to competition of food). Pair of mandarins (biota captive bred only as they are eating pellets) (also if you get wrasse maybe not again due to natural food foraging).
Personally speaking I'd go with the gobies and chalk bass but that is easy for me to say. I'm not one for a pair of clowns in that size display but it would work. To me they are a bit large and would take up space. Gobies I could fit more in same with chalk bass. Also most of these you can get captive bred/raised.
The long and short of it is really consider the size today vs tomorrow (maturity). I do not mean like tang police but rather things change as days go by. Whatever you do choose it is about the journey so have fun and enjoy the ride. The display is a nice one.