fish to add

I've been cycling my fish tank (25 gallon) with three zebra danios for about
six weeks now and I think it's time to add some new fish so I've got a
couple of questions:
1. Will zebra danios school with other danios species (for instance leopard)
or I should stick only to zebras. I think it will.
2. I also want to add some barbs later. Will they co-exist with danios
peacefully or not? Any other issues with barbs?
3. What other fish would you recommend?

Thanks.

Alex.