All of this is coming from a noob grower, on my third right now, but imo:

1) Go with the larger tent if you can. More room is ALWAYS better when feasable.

2)I don't see the carbon filter being a problem, however, even if it isn't optimal, you will have plenty of time to replace it IF you start noticing smell.

3)Idk. Sorry. I just got my first samples of FF nutes about a week ago. If you email them, they will send you free samples that would be enough to run at LEAST one grow of 4 plants, and maybe 2.

Hope it helps!