Also whats a good side dish to serve with mac and cheese?
Use the two types of cheese you have already. If you like the cheese alone then I know you'll like the cheese together!
Agree with Britt McDaniel - go with your tastes.
For a wide range, look at this post (college student, extensive mac & cheese resume) and the virtues of each cheese she favors.
I use medium cheddar and sometimes add sharp cheddar. Add a few drips of hot sauce for added zing.
I enjoy broccoli as a side with a bit of lemon butter.
two cheddars, gruyere, and pecorino