Hey cool to see another kuhli fan!
Ok, so for now in the 10 gallon, you can add three, they are very social, I have four in my 29 gallon, but I am probably going to add one more
If you are upgrading to a 20 gallon, I would suggest a 20 long, same footprint as a 29 gallon, just 6 inches shorter, if the kuhlis are going to be the only bottom feeders, then I would add like5-7 of them in there, but I would consider your substrate first, with sand, sometimes it will make a difference and sometimes it wont, because sand is too compact, I sometimes can't find my kuhlis and when I do find them, I just see their head peeping out if my gravel, I have a smooth gravel, because you have to take into consideration that kuhlis have very fragile scales
So over all, great hardy fish
Oh, and one more thing, if you can get someone to drive with you to the other store, so on the trip you can open the bag, stir it up a bit for oxygen, and then close it, but don't shake it too hard because you will stress out the fish
Good luck!