when upgrading valves for a high-output engine, can i mix valve materials? i.e. titanium intake valves with Inconal exhaust valves?
since i know there is a difference in valve weight, will this put stress on the timing cams and/or throw off timing?

I don't see any reason why you couldn't, although I don't recall ever hearing of anyone with this platform having a need to do so either. It's the difference between light and lighter, isn't it? Pretty sure Iconnel is prized for its heat characteristics, but why would the exhaust valves need it and the intakes not?

I'd suggest saving some cash and sticking with good old fashioned Stainless unless you're out to build a 420A for NASA or something.