It is true that all speakers in your home theater should be timbre matched. But there's a lot more to it than just brand name, model name, or even type of drivers.

A lot of aspects of a speaker can potentially impact the tonality or timbre of the speaker. The port if it has one, driver material, size, type, crossover topology, crossover point(s), cabinet shape and size,etc. Speaker placement and room acoustics affect timbre too.

Ideally you'd have 5 of the identical speaker for optimal timbre matching. Since this isn't always practical or feasible you generally shoot for the next best thing - 5 speakers with identical drivers. Results will vary on how closely these blend together. Failing that you shoot for close tonality - voice matching if you will.

The good news is that speakers can be voiced matched by design to behave similarly in the midrange where timbre matching is most critical. I think most manufacturers do this with their products now.
Ascend's different tweeter and woofers probably aren't much of a concern if they claim their speakers are voice matched. You probably won't notice the tweeter being different as much as you would a woofer. If response plots are available, you can get a good indication just by looking at the shape of the curve, take notes where peaks and dips occur. If they are considerably different across wide bands, you could have issues.

If you're really worried, buy 4 identical speakers for front mains and surrounds, and the corresponding center channel. As I said, I think most designers consider product ranges when designing now, so timbre matching is usually pretty good even if there are substantial differences between the speakers.

Don't worry too much about this...make sure your front 3 speakers are timbre matched at a minimum - follow the manufacturers recommendations. If you decide to get creative in mixing and matching, you'll probably want to test out your combinations, preferably in your own home.