Generally, you're allowed to switch to SL. If you take Math HL, then it's your 3rd HL, right? So you'd have to switch one of the other courses to HL.
I had trouble with my B language. I don't think I could take it at HL, and I'm honestly not holding my breath for a 6 for English A1 HL.. I guess it depends on your ability and confidence and how much time you're determined to put in.
I'm doing Math Methods SL. I don't know what's all covered in HL, but I know it's a lot more calculus and analytical thinking.
I hate coordinate geometry!! There's not that much in SL, I didn't think. You have to understand how functions work, but it's not a big deal. Simple transformations. And you get into vectors, but you don't even have to understand it if you memorize it

Doing physics helps with the vectors though =)
This thread is super helpful:
http://www.ibsurviva...-which-to-take/