Are you going to upgrade later for the harlequins? The minimum for them is 15g. They also prefer groups of 6+, the more the better
In a 10 you could have pygmy, hastatus, or habrosus cories. The habrosus are the most cory like as they stay mainly on the bottom while the other two like to swim in the middle as well as the bottom. These also prefer groups of 6+.
I'm not sure if I'd have both schools in a 10, hopefully someone more experienced with stocking numbers will touch on that. Maybe with good filtration and tank maintenance it'll be ok.
Snails or shrimp might be a good addition. There are other fish that could go in a 10 but I wouldn't add any more mid-top dwellers since the harley's already have that and would be better off with more.
I'm not trying to bash your tank or fish at all and really hope I'm not coming off that way. Just want what's best for them
Some smaller fish for a 10g would be: Peacock gudgeons, mosquito rasbora, neon green rasbora, celestial pearl danios, spotted blue-eyes/pseudomugil gertrudae, certain killifish, hara jerdoni. Others too that I'm not thinking of!