Join Date: Feb 2005
Bikes: Trek 5500, Colnago C-50
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"can I use a 8, 9, or 10 speed derailleur just as well on a 7 speed cassette?"
Yes, but try to find an 8 or 9 speed derailleur. A 10-speed would probably work but the cage would be on the narrow side for your wider chain. If there is a width problem you could run an 8-speed chain, maybe even a 9-speed chain. Do not buy a pre-9-speed Dura-Ace derailleur, the actuation ratio would not be compatible.
"should I stick with a shimano derailleur?"