as far as i know, fiscal sponsors are only involved to serve as a "pass-through" for donations and grants. you want to have a fiscal sponsor so that individuals can get tax-deductible receipts, and so that foundations have a non-profit organization to write a check to -- foundations almost never give money straight to individuals. the fiscal sponsor simply takes the money, subtracts about 5% for their own administrative costs, and passes the other 95% to you.
unless you have some other written arrangement that involves profit-sharing and distribution, there should be absolutely no restriction on what you can do with the film. good places to look into for fiscal sponsorship are: Women Make Movies, IDA, and Public Media Inc. These orgs are all specifically set up with a filmmaker in mind.
eventually, you can indeed set up an LLC too. yes, the startup costs are expensive but you can't open a business bank account without having some sort of corporation. and keeping track of business expenses mixed in with your own personal expenses gets real old real fast. good luck!