could I use this rear derailleur RD- 1050 (its made for 7 speed) with a 9 speed shimano drivetrain?
Is there any difference between a shimano 9 speed derailleur and an 7 speed one?
In theory, you should have no problem. The early 105 derailleurs had the same amount of throw per cable movement as the latest 9-speed.
In practice... maybe. The 7-speed derailleurs didn't need to move as far across the cassette as 8/9/10 speed derailleurs, so it could run out of travel even with the adjusting screws all the way out. As my friend the surgeon keeps telling me: "I dunno, let's operate and find out!"