Generally shouldn't have a problem using a derailleur that was made for less speeds, might not say the same about using a 10/11 speed one for a 5 speed freewheel.

This is assuming of course that you're using friction shifting. When you go to indexing land then a whole bunch of issues ensue.

My nuovo record rear derailleur that was made for 6 speeds at most is now running 8 with no problem, some have done 9.