You know, I normaly never make cardinal sin recommendations of other brand/type speakers for mix-matching speakers in a system. It is never really a believable soundstage, and the oerall sonic integrity of of the system end up being shot!
Your ears are never fooled ultimately into believing there is a seamless sonic field across the front, at the very least! And yes, I can certainly see where the other gentleman would suggest the NHTs for matches, which is definitley a close sound,yes!
However you really ow it to yourself to try to get the SCS3 if your going to stay with the Thiels!!!..anything else is really just doing it wrong!(actually another 3.6 in center is best). If you spent that kind of money already, finding a used pair of SCS3's is really what you should be doing! At the very very very least, you should buy the center!..and you can sacrifice maybe in the rears. Infact, I would always go with a quality 2 or 3 channel HT set-up before I'd ever trie to "hodge-podge"/"mix-match", and destroy the overall cohession of the system. 2 channel with a Pre/pro and a sub is still better overall than poorly executed 5+ channel! Infact most people can't even set up 1 or 2 speakers well in the first place, let alone missmatched multiple arrays! So, it's best to do it right.
Another consideration would be to just get 5 other speakers entirely for HT dubties, mounted to the wall if need be(depending on your set-up). Thn you can get lesser speakers for HT, which never has the refinement of 2 channel ultimately anyway. There's numberous HT type speakers out there that will do full glory to HT for very inexpensive outlay!...it's a thought.
Anyway, I'm familiar with the 3.6's, and you should STILL cross em over "smaller" on your processor!...and not run em full range for movies(sound weaker, distorted, and blown out for full range Dital tracks, at any level!). Definitely use a sub or two for below 80=60 hz or so...the overall sound will be much more dynamic and effortless, and non distorted.
So, my recommendation to keep it simple is that you HAVE TO buy the SCS3 for center at the very least, and maybe save up for power points in the future...that's how I'd do it. That match is perfect all the way around if set-up right!..why do it any differently?
Your using good enough equip, why not do it right?