I suppose some engineers computed that number. It is probably 75 lb per glide. Otherwise they would be useless under a table or chair. It seems reasonable. The feet and screw will definitely hold more than that. If the plastic nut inserts wear out over time you could easily replace them with T-nuts and they would be even more sturdy.
If the bar weighs less than 600 lbs. I think they would be fine.
Can you get a metal threaded socket, as I find the plastic splits too easily?
I would presume this is 75 lbs each. I have them on all my 4 leg chairs and I weigh way more than 75lbs and they are just fine after a years use.
Go online and look at the product and it will say if it is available at your store or if you have to order it.
I suspect that 1" is the maximum that the leg can be safely raised, based upon the depth suggested for the drilled hole. The glide is not that deep, so the bottom of the hole would be the stop for the threaded stem. The screw is a 6 mm machine screw. But, it is designed to screw into the plastic glide sleeve which requires an 11/32 drill hole 1"deep. I hope this helps you.
The screws are fixed into the base of the glides--buying separate screws would not do you any good.
I would not extend the felt pad more than a 1/2" The thread stem is 1" long, so it needs to be threaded in the insert by at least a 1/2" Remember these glides are used a levelers.
Yes, the insert comes with the glide. I don't know the OD of the insert, but you need to use a 11/32" drill bit for drilling the hole.