You've laid it out well and made great choices. My son has the Q200c center speaker which he ran with Q300's for about a year & a half. He only relocated them to rear speaker duty when I gifted him Q500's. The Q300's and Q200c do work fine together, no issues at all.
Because you're putting speakers close to a rear boundary, all the more reason to choose the non-ported Q200c and front ported Q300's. The Ascend Acoustics CBM170-SE's I had (until I recently traded with my son to get a pair of iQ10's back) are rear ported. I don't know if you were looking at something else. With the KEF, you could even choose white speakers that might blend better and achieve a higher WAF ratio. (Wife Approval Factor) That they're on sale right now makes them an even better value.
Big room. Even if not a fan of bass, many lesser subs will not have much impact in that room and if only one placement becomes more critical. I like SVS for sound, warranty, upgrade policy, and 45 day trial policy.
KEF R500, R200C, R100, SVS SB2000 sub, NAD T758 receiver, Pro-Ject RPM1 TT w/ Sumiko Pearl, Bluesound Vault2, Sony BDP-S480 BlueRay player, Samsung UN55JU6500F 4K LED TV, KEF M500 headphones, Sony CDP-CE500 CD changer.