I was just wondering if you could use a different vendors gpu for the passthrough. For an example, my main gpu is a 980ti, am i able to use a radeon 260x for the passthrough?
I'm pretty sure you can, but check the passthrough manufacturer's website.
Yes, you should have no problem at all.
The actual problem with PCI passthrough of GPU's actually occurs when you have two identical cards as although they will be in different PCI sockets and therefore addresses, the GPU's are identical and so are their identifier addresses. (And you have to end up patching some xen stuff into the kernel and all hell breaks lose!)
Anyway, yea no problem.
Puget systems has an older (but still very useful) example of this over HERE.
The AMD card should pass through just fine you will just need to set pci stub so the host will ignore the card and will pass it through cleanly.