any science to mix and match speakers for HT?

Ardent two channel audiophile wants to create entry-level home theatre. I wanted to create 5.1 system using my 13 year old Thiel 1.5s as front R+L and wanted to know if anyone thinks it is very risky ( for poor final sound) to just buy any decent used mid level audiophile quality center and rear speakers? I already have a decent mid level velodyne sub. Thiel has a rather high end center that is above my budget so I was looking at others.
thanks for advice
It is better if you stay with Thiels for the fronts, for timbre matching. Look at used MCS or an SCS 2 or 3. Not so inportant for the rears. I want to match mine but at the time they did not make a center channel.
The center should be of the same type (same sort of sound/timber as main front)
Or it will be bad.
The rears are not nearly as important. Though I would buy something from Thiel for them also.
while i understand that timbre matching is quite important, my experiment with thx active monitors for center and surrounds yielded some very good results, at least for me, since i did not have to acquire another multichannel amp in the process.

the outlay for matching center and amp to drive it was beyond what i was willing to fork out; ht is just not that important to me.

but since using the active monitors, i am now more inclined to even acquire another pair to use for the main left and right exclusively for ht while separating 2 channel stereo.

still more reasonable for me than getting a timber matched center for my existing mains.

if that makes any sense.
I run a hodgepodge of speakers. I only play 1 or 2 movies a week, but my 2ch is on, just about all the time. I have Sophia 2's,fronts.--A horn type for a center. Mirage in the rear. The pink noise ain't anywhere close to being the same--however it does just fine for me. I do BD with a denon 4308ci.
Ok, I never bought different speakers on purpose and the cost of the Wilson center is just over the top,4me.
I constantly think about a used 20k amp or 10k speaker cables--(or 17k mm opus wires).
So it ain't what people here think and I can assure you my HT don't sound "bad". However if I didn't have speakers on hand and had to buy some and the cost was something I could live with; I would stay with the same brand all 'round.
Have you tried just using your 1.5s without a center channel speaker and just adding surrounds? Unless you have people sitting well off axis to the sweet spot you may find you might not even need a center channel speaker and possibly avoid whole the matching hassle altogether. I did this for years and never missed having a center channel speaker. On the other hand, I've heard lots of 5.1 systems even with matching centers that sounded like crap for dialogue. Just another (and considerably cheaper) option fwiw.
You should be able to find some 1.2's or so for under $400. Buy 2, use 1 and put 1 away somewhere? That should do the trick? You really need the same voicing across the front 3 speakers. Rears don't matter as much.
thank you for the very helpful and interesting replies. hs