I think composition is ultimately a very personal thing. There are no rules. Keep in mind that many great songs/pieces have been written with very little knowledge of what is "supposed" to be "allowed". In other words, theoretical ideas never mattered, it just needed to sound good to the composer. That's the bottom line, right? Does it freaking sound good or not?

Of course, there are many ways to think about composing something...theory can definitely play a part, along with other concepts, methods and ideas. As always, explore and find what works for you.

If this all sounds a bit abstract...well, of course it does. This is art.