You can, within limits. Some species, such as bluegill, redear, and green sunfish get too large or aggressive to be safely kept with certain other species. If this is for your 36 gallon tank, I suggest you avoid the ones I listed. Better options would be:
Small to medium, moderately aggressive:
Longear
Northern Longear
Redbreast
Redspotted
Blackspotted
Dollar
Warmouth
Small and shy:
Orangespotted
Bantam
Very small and shy:
Banded
Bluespotted
Blackbanded
As a general rule, you can mix species within one of these categories. You should still watch closely for excessive aggression and be ready to remove fish that are too aggressive or too stressed out.