Yes you are correct, "timbre matching" is a fantasy unicorn for HT. However, your overall system has some very questionable allocation of resources.
Your are outrageously overpaying for those in-ceiling speakers and in-walls ... $439 apiece for speakers that will be doing a lousy 10-20% of the output is an incredibly poor investment, esp. since you're sticking yourself with a craptastic $200 subwoofer .
The R625A for $520/pr at Adorama is not a bad deal but keep your expectations very very low on the built-in upfiring module, it's just a stupid marketing gimmick.
The 504C center is the best decision in the whole proposed setup. However, you can get the 95% identical RP-450C center from Crutchfield for a lot less ($400).
Keep in mind that the center does 60-80% of the total HT output, so big hulking towers like the RP-8000F are completely unnecessary for HT use.
Have you ever heard or owned Klipsch speakers before?