Without knowing your functional requirements or casting process how on earth can we even try to answer those questions?
Foundry should be able to give you some idea of their process capability/typically achieved tolerances. Often casting tolerances are stated as a function of size. If you want to try and design the casting without talking to foundry - which given the questions you're asking would probably be a very bad idea - then google some foundry's and see which give some indication of their typically achieved tolerances.
If working to ASME stds take a look at ASME Y14.8. Y14.5 still applies to castings, Y14.8 just gives some guidance on castings specifically.
I tend to agree with drawoh that separate drawings is generally the best solution - but I have made single drawing work, especially when the foundry also oversees the machining and you're just buying a finished item. 2 separate drawings gives you more options though.
