i have to ask now
why shouldnt you mix them is it that they have diff levels of aggression or is it because they need different types tank envrioments?
That's two reasons. Some fish are more territorial than others and require more tank space. Some have different breeding habits that can interfere with, or be interfered with by other species. Water parameters and diet are also other factors to consider. Until you know the species you're interested in and the size of tank they're going into, there's no way of advising what you can and cannot mix; there are just too many variables to consider.
I would suggest sifting through our LIBRARY
and read some articles on various species, tank set ups and suggested combinations. The more research you do on these fish and how to keep them, the more enjoyable it will be when you do.