ANY derailleur will work with friction shifting. Also there is just about no limitation regardless of the number of speeds, BUT more modern derailleurs built for 7 or more speeds will have narrower cages (9+ speeds narrower yet), and the narrow cage can sometimes create problems with wider chains, especially when feeding at an angle. If you're running 5or 6s, try to buy a derailleur advertised for 8s or less, though if you find a 9s bargain go ahead.
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