I think, for the most part, like will only school with like, but fish that shoal will kind of stick together. For example. black neon tetras won't school with regular neon tetras. But different forms of tiger barbs (green, albino, etc) will shoal with one another sometimes.
Keith & Ashley
65 Gal - Freshwater Community Tank
55 Gal - Mbuna Cichlid Tank
10 Gal - Betta tank