I know dewalt allow some mix and match between their batteries. For example, you can put a 12v in a 14.4v drill. However, you won't get anyone elses to fit.
If you get a combo kit, you'll want two batteries. I lived for 8 years with a dewalt 14.4v drill with a single battery. It wasn't bad, but every now and then I wanted a second. I wouldn't consider buying one seperatly as they almost cost half the price of a new drill with two batteries!
If you are 12v or less, they will die faster. Also, some of the newer ones have lots of power, but drain when unplugged. If unplugged for 2 days, you can lose as much as 20%, yikes! By the end of a week, half your power has gone!