Mechanically it should work, it is the shifter that will be indexed to 7 speed. Just set the high and low screw to the correct position and then tension the cable.
The problem will be that 9 sp and 7 sp use different width chains (7 sp being wider), so you may (probably will) get problems with the chain on the jockey wheels and cage of the derailleur and also the cassette sprockets.
I'm sure you can still get 7 sp derailleurs but maybe not that high quality like you have found. S-RAM do a 7 sp.