Russianjoo,
if you are competitive for anesthesiology then I doubt getting a prelim year will be super hard, although some of the IM ones are sought after. I think if you have enough programs on your list (and do research on which ones might be very hard to get vs. less hard) you should be O.K. Transitional years are notorious for being hard to get, though. Surgery prelim years are known for being kind of heinous and thus less hard to match into, but I guess it all depends on what you like to do all day...
I don't think you HAVE to write a separate personal statement, but it definitely wouldn't hurt. I think adding one paragraph on to the PS you have for anesthesia would probably be adequate also, but since I'm kind of OCD I'd probably just write a separate PS for IM prelim years and go for one of those.
Usually I think there are some unfilled surgical prelim years and maybe a few medicine ones floating around as well in the scramble. Not that you'd ever want to do that, but it's always an option.
It sounds like you've set yourself up pretty well to compete for anesthesia if you'll have 2 anesthesiology letters.
I don't really think that you need to have people write letters specifically saying why you want to do a prelim year (in med. or surgery). They pretty much know you need it to do anesthesia, and as long as you can explain in your interview, and maybe a little in your PS, why you want to do THEIR prelim year, it should be O.K. Getting LOR writers to write 2 separate letters might be hard to do.
