Well firstly, the best subjob for a BLM is WHM (and 'maybe' SMN, but if you're going Tarutaru it wouldn't matter I don't think).

You can change your main and subjob whenever you feel like it. =) So it is possible to get every single job to level 40 (although will take a little bit of time). I'd suggest just leveling your first main and sub to level 40 and see how things go.

Don't be afraid to try out new jobs though.