My bike is mostly all Shimano components, but I have to replace the derailleur and I hear Campy parts have a longer life. The Campy Centaur and Shimano 105 deraileurs are comparably priced. Besides the fact that I'll be limited to friction shifting, and won't be able to do index, any reason not to use a Campy 10 Speed Centaur derailleur with Shimano shifters and a 6 speed freewheel?
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
As long as you are using friction shifting, you certainly can use the Campy rear derailleur with any freewheel or cassette as long as the large cog is within the derailleur's capacity (29T for a Campy rd). A 10-speed rd may not accept a wide 6-speed chain so be sure it will clear the chain you use.
The Peter White web site is discussing the use of a Shiftmate to make different indexing systems work together. They work very well but none of them are designed for 6-speed anything.