Not looking forward to finding out how limiting my girlfriend's F55 Mini (5 door) will be in a few years time.
You'll be fine, we coped with my wife's R56 JCW and that fit her, me the baby (admittedly not rear facing but quite bulky Britax trifix car seat), Quinny three wheel buggy thing and the various other baby accessories that you have to carry round. You just have to get creative with the use of space.
Yes it would have been 'easier' with a bigger car but you don't need a massive thing just because you have a baby.