I use four retainers..before HW came out I used three (so two free and one bought). My current 4th retainer only holds HW gear.
Sell everything you don't need. If that is through a vendor or through the MB, doesn't matter. If you think you can make money use the MB.
I'm not sure about "leveling all" of one type of class (only one I would recommend is Disciple of Land since there isonly three). When I level my crafters I find that gear is the very least of my space issues... most of the space in my inventory is given up to ingredients and materials. And for DoW or DoM,.. well personally I'm not leveling them all (changes on the person of course, some people NEED to have everything leveled). Though to be fair I'm done with almost all low leveling I want to do.

Gil is useless until you really, really want something from the MB. (Like how I just spent a total of six mil for mats for my Company Tabard... bye, savings).

Also yeah fishing is. You won't make money from fishing until you level CUL to desynth the big big fish.. once ina while you'll find a few "worth it" but, not many. Almost all of my profit has been in mining.

Also you think you have an inventory problem now? Just have a bunch of non-stackable collectible sitting in your inventory (I'm talking about at least a page) because you can't turn them in yet....