Bicycle Mechanics - An Ultegra GS vs ??? question

I've spent hours digging thru old posts, but I have a question... please.

I have zero experience with Shimano. I'm C&V, and old stuff. Campy or Suntour. Friction shifting.

Question regards the Ultegra RD-6500 GS.

I've been offered one of these for a reasonable price, and I'm curious. It would replace a Campy Rally RD. I would be using either downtube or Suntour barcons on a 7-speed freewheel rear end. Someday, I might get a bike with a wider back-end, but not yet.

Are there any improvements in shifting?

Is the RD-6500 GS a particularly worthy RD? Was/IS the RD-6600 or RD-6700 worth skipping over this option?

I would appreciate hearing from those who know this RD.

Thank you.

It is a great rear deraileur like or replaces Shimano 600 and designed for 9 speed rear wheel and the 66 and 67 are 10 speed rear deraileurs for 10 speed rear wheels. I would keep the Campy and it can run 6,7,8 speed just fine.

It's a nice rd. You may prefer it over the 6600 or 6700 because with those 10 speeds the cage is more narrow.

Gentlemen,

Thank you. Very helpful. Especially to understand the difference between those models.

I'm not eager to dump the Campy Rally, but curious to try something a bit more modern. Maybe even to try indexed shifting, which I've never used before.

Guess I'll give it a go.

Thank you, again.

Index shifting will have certain compatibility issues to consider.

Yes. I presume new shifters will be required. Beyond that?

I have some NOS SIS shifters, one pair rated at 7-speed, one pair at 8-speed. Bought them back in the day. Would these work? I presume the RD would have to have it's limit screws set to not throw the cage into the spokes, of course.

Am I missing more?

Thanks.

In my experience it will work with 7 thru 9 speed, index or friction.