I think as long as the caterers and venue know and are cool with it, it's fine; it's just a business arrangement and as long as you are upfront and not breaking anyone's rules, why not?
But logistically, yeah, it sounds like it might be challenging. If you brought the caterers together they might be able to work it out. Or, start with the caterer you want most, and see if they have any ideas. Like, if the Thai caterer told you upfront that they can't do more than 100 people and they won't work with anyone else and they won't provide non-Thai food, that would have a big influence on your planning--either you conform to their specifications, or you don't use them.
I think it's something you could definitely investigate during the early planning process, but be prepared for it to not work out exactly the way you'd hoped. A caterer who can handle bigger volume and do a broad range of traditional and fancy dishes might be best for the reception. You could do Thai for the rehearsal, but if the people closest to you are also people who aren't adventurous, they probably won't be too happy with that option. I don't think I could get my dad to even walk in to a Thai restaurant, for example.