Can I connect a heater and air conditioner to the same circuit even if the combined loads would be over the breaker rating?
Yes, if you never draw more than the max output of your generator then having an A/C and frost protection heater wired to the same panel would be ok. However, that doesn’t allow for human error. I don’t know what code is for generators, or if they include internal fuses or circuit breakers. (If your crawl space heater is thermostatically controlled then you would have to really TRY in order to run it and the AC at the same time. You’d have to run the heat to warm up the house in sub-freezing temps, and then turn on the AC and lower it’s setting until it switched on, which would be a really dumb thing to do.)
It would be safer if you were to wire a master breaker into the generator that would trip if you attempt to overload it. That way if you screw up and turn on the crawl space heater and the A/C at the same time, you’d just trip the breaker. (Or if you just run the AC, a toaster, and a hair dryer at the same time. It’s not hard to exceed a modest current feed like 50A with resistive loads and/or big motors)
What is the max power of the generator? Max current used by the AC? By the heater?