I'd note in terms of "green village", that pretty much describes most of Cambridge. Cambridge is, in a sense, a very large green village. So that would be true at literally every college. Such is the nature of the city...
As above, it's pretty common for postgrads (particularly PhDs) to live out of college after first year. In fact I think you'd be more hard pressed living IN college after first year, since the colleges have limited rooms available and prioritise undergrads generally as I understand. Also in general for postgrads life is centred around the department, not the college, unlike undergrads. The college is just where you sleep (while you have a room there) and and have pastoral support being provided).
Additionally as above there is generally no mechanism to transfer colleges. Extremely rarely students may be permitted to transfer if there was some kind of extraodinary set of extenuating circumstances (e.g. major trauma involving someone else in the college perhaps, and even then maybe not...). It's certainly not normal or common and there is absolutely no guarantee you will be able to when you go there.
In any case, there are regular buses in and around Cambridge so cycling isn't a necessity, and I expect the are additional transport possibilities through the accessibility office if needed. Also while technically you're not allowed to have a car in Cambridge as a student normally, I know quite a few postgrads had one there anyway.
Also you can regularly find rooms from the federated institution colleges offered on the internal housing finder thing they have, and sometimes from the "regular" colleges too. Not to mention the private rental market otherwise. There are additionally a number of other accommodation sites (Causewayside, the West Sites, etc) which are exclusively for staff and postgrads.