I have seen tanks with chain loach (about 5 of them) with 1 zebra (b. striata) and they get along fine, tbh they dont really interact.
I think most types go well together, but skunks can be aggressive or are the most aggressive of the lot.
I could only really see the size of a tank to be reason for territory squabbles and yours seems fine you could probably get another zebra and 3 yo-yo's. Although yo-yo's do get quite a bit bigger i think it wont matter as they like the interaction (i think, im no expert lol)
I would get some more zebras if i were you, i have 3 in my 30gal and they love it, loaches should be kept in groups of 3+ and you could have 5-6 in your tank.
Although i would keep a bigger amount of zebra's personally to make them feel more safe as the yo-yo's get bigger.
Yo-yo's are great to watch, My brother has 3! in his 20gal they are more than happy and are always out and about.
However 20gal is s bit small for them and he would love to get a bigger tank to see them really flourish. HTH