Hiya,
I’m pretty sure that C7 files will open on C6 versions and vice versa, the only issue I can see arising is if you’re using any full version only features, but if you take this into consideration when you prepare the project for sharing it shouldn’t be a deal breaker.
Have a squizz at the product comparison page on the Steinberg website and see if there’s anything that you’ll need to be aware of.
Cheers,
PC