any science to mix and match speakers for HT?

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.

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