Honestly, nothing on swingweight unless you have a Babolat RDC. And sadly, yes, swingweight is the key ingredient to making 2 rackets feel the same. But different people have different preferences. As you know, it's not always possible to match EVERY spec perfectly everytime, so pros generally have a list of which are more important to match compared to the others. Some like balance the most, some like mass, some like swingweight. Sampras was a tough guy to work for because he wanted all 3 the same. Good thing his rackets had so much lead added to them, otherwise it might have been a serious headache for Nate Ferguson.
I'd start off by trying to match them before I add weight, then I can add the same amount of weight to all of them. Or, if I know which racket I like, I'll treat any racket heavier as if it had a little weight already added somewhere. Then I just calculate how much less weight I have to put in a general location to have it come out about the same. And any racket that's lighter, I add weight so that the specs are matched. Sadly, it's tough to match the swingweight without a Babolat RDC machine unless you're Pete Sampras.
Depolarized frames are far better on volleys. Not even debatable. lol And for the high one handed backhand, the polarized frame is tricky to use because you need to be able to apply heavy spin to control the shot. However, I don't have a problem with that personally... I have no idea why. Polarized frames are overall better for me. But you can get some easy pop with a depolarized frame. The racket does all the work.
As for making a list of stock polarized and depolarized frames... The only ones I know are K90 being relatively polarized, and the n90 and PS90 being depolarized. I heard the Sampras racket is likely depolarized too, which makes sense since what he uses on court is a heavy, depolarized beatstick.
As for whether he played better with the ProStaff 6.0 Midsize than with his current racket(s), I have no idea. Never seen much of him at the time other than 2001 Wimbledon. He did hit some amazing backhands, but they weren't off high balls. Back then he sliced a lot of high balls if I remember correctly.
As for what Guga used, I'm not sure. I'd guess he used a polarized setup. But during his time, they were a rarity.
I can't find any pictures with any lead on his racket.
Perhaps he went stock?
Depends on how much you want to improve stability. If only slightly, then 3 grams total is good. If you want something that's rock solid, then 5-7 is a good number. If you want something that's like a wall, then go into the double digits. Haha.
How long depends on how much you're adding. Some people like it longer to spread out the effects of stretching the sweetspot. I personally like it longer. Looks nicer, and it requires less layers.