Can I use a Suntour Cyclone GT Rear Derailleur for a 10 speed Campagnolo cassette? I'm building up a road bike and have a wheelset with a rear hub that has a Campagnolo Chorus 10 speed cassette. The front derailleur will be a triple and I'm looking into the possibility of using a long arm friction shift Suntour Cyclone GT rear derailleur.

Is there any reason I can't do this? Will the Cyclone have enough adjustment range? Will I need to replace the derailleur cogs with thinner ones for the 10 speed cassette? I've heard the older derailleurs like the Suntour have worked with 9 speed cassettes, and I may respace the Chorus to a 9 as an option.

This is one of many questions I have to ask about this bike build, but thought it best to focus on one question at a time.
Campagnolo 10 speed cassette is only 0.6mm wider end-to-end than their 9-speed cassette, so as long as the 9-speed setup still has that much adjustment in the limit screws you should be fine on that count.

The jockey wheels only care about the inside width of the chain which is 3/32" on pretty-much all derailleur gear systems that have ever existed, so you won't need to change them for that reason.

I would try it, if it doesn't work you can always fall back to your 9-speed plan.