I have an EFXR6, and i just bought some 6.5" focals.. they will fit, it will just require some 'installation' work
You know in your EL XR6, where the side mirror adjuster is on the drivers door? Above that there is a grey plastic panel that looks like a sail. I mounted my JVC tweaters to that and ran a wire down the inside of the door to the crossover.. It's worked well so far, and they recommend you have the tweaters 30cm's away from the woofers, and it's roughly right..
About what splits to get, we need to know your price range and intention. Do you want alot of bass and loudness? Or good quality sound.. Do you wanna be able to build on your system? Or do you want to get the best now and save time.. ?