So guys what would you do, am sitting here looking at my camera stuff, and I really want to get in to the mirrorless world BUT
I have the canon 750D/T6i with a sigma art 50mm and I kinda want a telephoto lens to, so I have looked around, and I can find some pretty nice once for around the same price as the r10 with an adapter on the used market.
Or I can get the r10 with an adapter. Plus if I get the r10 I can save up for the RF 100-400 canon lens. Plus plus I can sell my 750D/T6 for little under half the price with a lens( not the sigma art) to my buddy. So the question is would you get the r10 with an adapter and explore the mirrorless world, or would you get a nice quality telephoto lens
Why do you want to join mirrorless? What does your t6i not do, or how does it fail you and how would going to mirrorless make things better/easier for you?
If you do not have a clear answer to that question then it sounds like you simply want a new toy. Which is fine but realize a new camera when your old one does everything you need is not going to automatically improve your photography or change it.
Generally it is better to invest in lenses that open up new opportunities or provide better image quality vs. buying a new body unless your current body is limiting you in some way.