I have a backup generator with a switch panel that I want to wire. Trouble is that the circuit sizes are not the same. My generator has a 50, 40 and multiple 20s, but my service panel has a 60 for AC1 and 35 for AC2 and a 30 for the well. Can I swap out the 50, 40, and 3-20s in my switch for a 60, 35, and a 30? The master breaker is a 70 for the switch panel.

Sure can, using the same brand circuit breaker.