Unless, you just has to have the 500 then I would get a 460 if the X frame is your thing. The 460 allows for 460, 454, and 45LC to be fired versus the 500 and only being able to use proprietary ammo.
I'd be careful shooting too many "short" cartridges (ie., .45LC/.454) in a .460 chambered revolver - especially the .454. It is a high pressure round, and can eventually create a "ridge" where it's case ends, which can make .460 cases difficult to chamber, and possibly create pressure issues with the longer case.
For this reason, Freedom Arms does not recommend firing .45LC in their .454's. Granted, FA has tighter tolerances than S&W, but it's still a practice to be avoided when possible. If you're going to do it, clean the chambers VERY well afterward with a brush.
If you want to fire .45LC's, most would be better off buying a (relatively) light .45LC, not shooting them in a 4+ lb. .460 revolver, imho.
As to the rest of your suggestion, the .500 doesn't use "proprietary" ammo - it's made by every ammo maker on the planet.
Regarding the 6.5" .500, I have one of the PC versions with heavier barrel, and 360 degree porting, which I believe is a different setup than the original poster was asking about.
Still, I think the 6.5" length balances better than any other in the X-frame. I have the 2.5 and 4" guns as well. Have shot the 8 3/8" but didn't care for it.
Recoil on the shorter barrel (2.5 and 4") isn't as bad as you'd think, compared to the longer ones. The velocity drops, and this seems to offset the shorter barrel/less weight in recoil. The 8 3/8" seemed just as bad to me as the 4" with Corbon 440's, for instance . . . probably because of the 200 or so fps higher velocity. Just a guess.
see ya