David offers a useful piece of advice (among many) about a-z filing and I'll paraphrase here...
...put everything into one a-z file or easy retrieval until one or more subjects gets to the point where it may need its own drawer or filing cabinet...
Kate, in your situaiton of mixed job and business start up this is probably the way to go. Initially you'll have a few papers about business setup but this is going to balloon quickly. Just move your business filing into its own drawer then cabinet and then its own Trump Tower when you are a publishing mogul
As to financials just one file marked "F-Financials" simply doesnt work for me nor I suspect a lot of people. Life finances are way to complex for that but the a- z guide that GtD gives works just fine and Jason's advice about asking the where / how will I retireve this from the filing system in the future is a key question you need to ask.
Filing is a 'work in progress' and I always find myself going back to GtD basics on this one. But I am becoming way more discriminating what I keep and WPB file 13 is getting used a lot more.
If all this is too hard you could always use my original
Biblical filing system
'Seek and ye shall find'