I would say that 30 buyins is a little too loose with your bankroll management, but it is only $/£45 so if you don't mind massively about losing it and you think that you have a decent skill edge in that format then you should be okay.
As for moving up and down, that's exactly the point. Say you win a bit and get up to $/£80 or so (and you still want to stick with 30 BI) then you can start to mix in a few 3.50s, and if you get up to $/£105, then you can play solidly 3.50s. If you lose, then you need to do the opposite and move down a bit (I guess you would have to play a few of the 0.50 turbos.