500 free - what stroke/s are acceptable? Can I mix them?

I'm doing some 500's, have always done what most call freestyle. Can I ever mix in other strokes? Do I need to do the same stroke throughout? I'd love to do some backstroke during it. Am also aiming for an 800. I've read the rulebook and either not understood or not seen an answer to my questions. Thanks! I've only been racing 3 years. Debra, Ohio :)