I'm moving this question over here from the Road category. I have a 1991 Trek 1200 with the RX100 groups set which has indexed friction downtube shifters. I am trying to turn this into my trainer bike for rough weather that I am trying to spare my tri bike. You can get old stuff on eBay and the like, but I am having trouble figuring out exactly what parts from the late 80's or early 90's will fit on the bike. I don't think Dura-Ace is compatible but I'm pretty sure that Ultegra and down are. Any advice?

More or less correct. Anthing with the same number of speeds should be OK except for Dura Ace.

Why whats up with dura ace?

For a rough bike, I wouldn't use Ultegra. I'd find some beat up 105, LX, or RX100 and use it.

Early Dura-Ace had a different specification for the amount of cable pulled per click. Shimano brought all of the lines back together as far as being compatible with 9 speed.

See here: https://www.sheldonbrown.com/dura-ace.html