Yes, you can,
can i use water base paint over oil base paint
Yes. Clean and degloss the oil based paint, apply an oil based primer, then apply the water based paint.
Only if the 'flat paint' is water based too.
Not a good idea, better to use oil based primer.
depending on the surface finish of the primer. Noirmally its matt in its appearance and will take a water based gloss. However, a primer should have an undercoat applied first unless its a primer/undercoat.
Yes, you can
You can paint water base paint over an oil based primer after allowing sufficient drying time. Check the instructions on the label because drying times can range anywhere from an hour to 24 hours.
You can always paint latex ( water based )over an oil based paint as the latex just dries on the surface. However don't paint an oil based over a latex paint. Oil based paints need to absorb into the wood or other surfaces or it will just dry and flake off. For more information on the differences and when or not to use a latex or oil paint please see the following article. . http://www.ehow.com/how_4713170_between-latex-oil-base-paint.html
It's generally much better to use water base primer on drywall.
yes. However, you cannot use an oil base paint over a latex primer. If you change your mind on the paint, you must strip the paint before you use an oil based paint on the primer.
Yes, if the primer is oil-based too (alkyd is good too)
Unless you used a primer in between it will look like it's slipping off.