1-A 20 or 29gal wood fine for kuhlis.
2-It is best to keep them 3 or more as they will be more active, if you keep one it is very likely you wont see him 90% of the time.
3-I would say keeping Angels with them will be fine as they are'nt aggressive.
4- As far as i know they do a good job of cleaning up, that is besides algae.
5- Sand is the best substrate for them as they can get scratched on graveletc.
I guess you could have a whole group of kuhlis if you like as long as you dont exceed the stoking limit and you could put an otto in the tank for the algae.
Hope that helps.
