You don't have to use its own cleanser. I don't like its own cleanser that much anyway. I've tried it with Cetaphil, and Philosophy Purity made Simple, both are equally good. I would choose a non-foamy or a cleanser that foams less because I don't like to have foam all over my face as I am using the brush.
I've used my last brush head for about 6 months and it was fine. Recommended is 3 months, but like toothbrush, it is your own judgement. It also depends on how frequently you use your Clarisonic.