You can buy thai sim cards in every 7/11 so you can stay online during your stay. At the airport they have some spaces where you can buy them, I think it's TRUE and DTAC stands just close to the exit (either same exit as taxi or one level below).
Grab is Asia's version of Uber. When I arrived end 2017 you had both but if I'm not mistaken Grab purchased Uber and now there's only Grab. It is currently being (or will be) legalised and for that reason taxidrivers have been protesting outside Ministry of Transport lately. There are reported cases on taxi drivers having beaten up grab drivers. Regular taxi is cheaper than Grab taxi. Grab motorbike cheaper than regular motorbike taxi. Obviously grab has the advantage of the customers not having to explain where to go, which can be tricky with regular taxi. I use Grab motorbike every morning to work. Home from work I use the bike taxis standing there to support them but also becauss, as almost every taxi driver, they're big muay thai fans :).
I don't think you need to worry about being rag dolled or exhausted. If by this you mean being swept and thrown a bit yeah that happens (for fun but no serious) but if you are an MMA guy you already got that advantage. They will pair you up in clinch and it's either gonna be with a trainer or student and it would be a dick move to go hard on a new guy.
Thai gym trainers are used to westerners being tired just having arrived. The big risk is if you say you are tired but the trainer can see you are not (they know for sure) they might not respect you. But they will see if you are super tired and really try. If you are new they not gonna try to break you. Except padrounds and to some extent clinch training pace will be up to you. Personally I'm more concerned with getting easy padrounds and not being pushed at all. It happens I get a trainer who thinks I'm not strong enough and will give me short padrounds with very few speed kicks and never tell me to do pushups or anything in between. That hurts more than being so exhausted from padrounds. If you only stay 2 weeks unfortunately there's a risk trainers won't invest in you too much to push you too hard.
Just be yourself, make sure to give an effort and enjoy your time. Let me know when you're there I look for you during the morning run .