DON - Yes, there is such a thing as too much weight. You want to shoot for between 5 and 10% slippage, but on loader work don't ask me how you figure that out! Your weight distribution changes so much from loading the bucket to moving up to where ever you intend to dump the bucket, to returning to whatever pile of "STUFF" Your loading.
All totalled, my Super H weighed 5250# with no loader, fluid and one pair of wheel weights. I guess the loader weighs about 1200-1400#, and I added a second pair of weights 5-6 yrs ago, so I figure it's around 7000# now, and up to 7500# with the tire chains. The '51 M loader tractor weighed around 7000# (no fluid) before SON & I added 900# of weights 4 yrs ago, and with it's chains, probably goes 8200-8300#.
Like Steve B listed in his post about his fork truck attachment for his loader on his 1872, I don't think 400-500# is too much for a CC loader tractor to handle depending on how it's attached to the tractor. His 1872 is an SGT, longer so the weight being further back helps him with stability more.
Yes, I have inner tubes on ALL the rear tires on all my tractors. The only tires not tubed are (I think) the frts on the 982, I've never had one of it's frt tires off the wheel.... yet.
Also, even on garden tractors, you should check tire PSI every couple months, they lose air prssure with colder temps just like car/truck tires.