You will get a lot of good advice from people far more knowledgable than myself, but I was recently told you are ok as long as you don't plant things from the same family next to each other (like different kinds of squash,or whatever). The only risk of cross pollenation is in memebers of the same family. Othedr than that, just take into account what the plant likes to make it happy. I just found this ouit the hard way-I alsways plant marigolds around my veggies, but my new square foot garden is so lush and wonderful, my marigolds are dying!
Let's see what the pros say.