I'm going to replace the chain, and cassett on my Trek 7000 but the chainring has a sticker IG chain ONLY? Can I use a SRAM PC-850 without issue, and is a HG50 the cassette I should look for? The current (original) chain is marked (Shimano IG Narrow). Also, the cassette is a 7spd so I assume the SRAM 8spd chain is the correct choice?

Thanks Capecodder......

I use a SRAM 850 cassette and chain on my mountain bike, which has a Shimano STx crank marked "Shimano IG only" It works great.

Is the spacing on the SRAM cassett correct for a Shimano STX rear derailleur?

IIRC, the IG chain is just slightly narrower than the HG.
I had an 8s IG setup for years on my mtn bike before I upgraded it to 9s.
I used to use SRAM 8s Powerlinks with Shimano IG chains on it, and I've read that would not have worked with the HG chain.

A little info about the IG here: https://www.sheldonbrown.com/k7.html#interactive
I believe only the chain & cassette were different on IG, but the center-to-center cog spacing was the same.

I use a Shimano Deore LX rear derailleur, it works great. I did notice a slight difference in spacing when I had a Shimano cassette on one set of wheels and a SRAM on the other, but now that I use SRAM on both, it shifts fine.