Okay, some good questions. I'm going to deal with them in a slightly different order to how you asked...
I'm not sure what your understanding is of speaker drivers or why you think the centre speaker would necessarily have to have three tweeters. This doesn't rule out a centre speaker design from having three tweeters of course, but it is very unusual. The centre speaker is producing sound for its own channel, and not for all three front channel speakers.
As for having the same or differing designs across the front three speakers (left, centre, right), the ideal configuration is three identical speakers. This even extends to all three speakers being oriented in the same way. Clearly though that has implications for practicality and aesthetics: A horizontal design is often the preferred choice for a centre speaker purely because it's a better fit with a TV screen. But that then has an impact on the sound. Just lying a speaker down like that will change the dispersion pattern and so also change the tone of the sound. So that's part of why you see centre speakers with a different design to the left and rights. If you also look at the content of the sound from the front three speakers, nearly all the dialogue comes from the centre, and it's providing upwards of 70% of the sound in the front 3 channels. The front L&R are mostly contributing effects, music and some panning effects. So the centre is the hardest working speaker. You'll often find that with smaller 5.1 speaker kits that the centre is the beefiest just because of how much work it does.
Where you'll see exceptions is in kits designed with LCR speakers. LCR stands for Left/Centre/Right. This is the same design speaker that can be oriented vertically or horizontally without changing the tone significantly. This has been done by a number of manufacturers - e.g. Tannoy (DC4 LCR), B&W, KEF, Monitor Audio, M&K, and other premium brands.
The Monitor Audio MASS system follows this LCR principle. So you're looking at a speaker system that follows the highest ideals of front channel design principles.
If you'd said you had no restrictions on where the speakers could be placed then yes, it would have changed my recommendation. I would have been talking about stand mount front speakers away from the front wall, and some wall mounted dipole surrounds. But that's not the case for you. You need wall mounted speakers, and while basic compact bookshelf speakers can certainly be wall mounted, there's very little mileage upgrading them to larger standmount speakers in the future if you're still expecting to have them wall mounted. So I stand by my recommendation of the MASS system both as an appropriate solution with your 636, and I think they're good enough should you decide to upgrade the amp in the future to something in the £1000 range.
In the end though this is all your choice. You might fall in love with some different speakers or change your mind about making space on the floor. It's up to you.
