I put a 3200 lb. load in my stock GM 2500HD and there was no squat at all. Later added a Superspring on each side for heavier loads. Even a 1/2 ton should be able to handle 1000 lbs. without needing air bags.
Air pressure will increase with ambient air temperature and with elevation. If I was planning to run at 50 PSI I would want a gauge that could read up to 100 PSI.
Important also to have separate fills for each airbag as you do not want an air bag that is supporting a heavy load to be having its air pushed over to the air bag on the other side that has less pressure on it in turns. Not a problem when towing a trailer but can be with a very heavy load in the bed.