Yes you can. Latex refers to what base the paint is. Latex is water based and therefore can be cleaned up with water. Oil based paint is cleaned with a chemical. Flat refers to the sheen of the...
The question is unclear. There are three types of paint by base: oil, water, and epoxy. None of the three can be mixed. Rather, one can mix the any two or three of them, but the result would be...
Sure, exterior quality is better because it has to stand weathering influences. But only if it's the right kind (don't put latex over oil based pain, unless you bridge it with shellac)