Originally Posted by
rufvelo
It gets complicated but yes, usually the rear derailleur has a lot of capacity. Problem is thickness of chain etc and the host of components you'll be changing - better get a 9/10 speed group.
Going to have to disagree with this.
If you're going to use Shimano STI shift/brake levers, you're going to have to get a Shimano SIS/STI compatible derailleur. Good news is, they are plentyful and cheap! If you're going with 10sp STI, then any new Tiagra, 105, Ultegra or Dura Ace rear derailleur will work. You'd likely be able to use older 9, or even 8/7/6 speed with little issue. Haven't done it personally so I won't say it
will work. Going 10sp on an older bike can open up a whole can of worms if you're not careful. Giving us a bit more information to work with will help get a bigger picture of what you have and where you want to go with it.
I've got a Campagnolo 10sp group on a 86 Serotta steel frame, works great, but it is a complete 10sp group, not a mix of new/old. Chain widths, front derailleur cage widths, lots of things are slightly diffrent as you get from old 7/8speed into 9 and 10speed.