best format... realistically a non compressed format (such as tga) with high bit rate (more colour range) when you save it, although the scanned image should be imported in a 'raw' format prior to saving it. I have this vague recollection that you'd be ok saving with a photoshop file too....
yeah it should be, don't see why not. Basically all an uncompressed file does is store every pixel scanned with the 'exact' colour the the scanner picked up rather than 'blocks of colour' like in say a jpeg when it's compressed (why you get jaggy edges etc).
Bare in mind 600dpi scans can hit several hundred megabytes in size uncompressed and it only goes up from there if you go higher res. Also don't bother with interpolated resolutions it's fake data so you might as well use photoshop to increase the resolution if needed later on.
Surely it depends what you're scanning in. If it's pencil or line work, scan in as greyscale. If it's a painting or photo, then scan it in colour mode.
I usually scan in at 600dpi and then reduce to 300. You will be guided by your scanner's software. Your 'format' will then be decided by what
you're going to do with it afterwards and what you save it as, etc. If it's staying as a sketch I stick with jpeg, if it's going to be turned into something else
then it might be a Tiff etc.