They will cash in your modules so that:
1) they prioritise your maths grade first. (You won't get BB in maths + further maths if you can get A in maths and a C in further maths.)
2) they move around your applied modules as to maximise your maths + further maths grades. (they try to give AA after all
) However this is not possible if you do different exam boards (you can still get an A level maths with OCR AS and edexcel A2 though, but your chances for applied modules being moved about are worse than with OCR only or edexcel only). Also, there are combinations not available, like S1M2 or M1S2.
It is OK to do maths from 2 different boards but not advisable due to the reasons mentioned above. You could do all A levels to get rid of resit, however, you would have to include everything in UCAS and I think this would be unnecessary effort. (Also, you'd have loads of exams)
From what I've seen based on your posts you don't need tuition from some sixth form college teacher. If you give yourself enough time, you can do this on your own. Teachers will accommodate themselves to weaker students in college, and will only keep you up. You can do more than 6 modules as well and the best 6 will be taken into account, however, I think it's better to do 6 and concentrate on them properly.
However, if you don't do physics next year and you'll only do maths + further maths, it might be worth doing Edexcel AS+A2 in one go. It is easy to get A*s with edexcel, even in further maths
Feel free to ask anything else