I've recently just finished my initial setup of a new system (with a lot of help, novice level). I've read that for aluminum a starting point would be 21k rpm with a 91 ipm feed rate. I ran the spindle autocal, and I'm curious what the next step would be. Is there a guide to choosing spindle rpm/ipm?

There are programs that calculate feeds and speeds, like GWizard or HSM Advisor.

Without knowing the type of aluminum, the exact tool, and the type of machine, it's impossible to say if those numbers are even close.