I wouldn't bother with 'fit', especially if one of your concerns is being from a low-income background — there is no such thing as a Cambridge college where most people won't be relatively well-off compared to most of the country, let alone outside of it. Instead, I'd concentrate on finances, especially rent and potential for awards, bursaries and so forth, since these vary massively between colleges and will mean the difference between actually having the time and energy to find people you 'fit in' with and constantly being anxious on account of your finances — the difference between a 'posher' college with an enormous endowment and lands it's had almost a thousand years to profit off of and a 'friendlier' college with a more moderate endowment and more modest lands can often, at least in my experience, be more or less the difference between thousands of pounds in tuition, hundreds of pounds in rent, and numerous awards given out based on academic merit or so that students might get new books, equipment, opportunities to travel and so forth. As someone from a very low-income background, and no carers to alleviate the burden or provide a residence outside of term either, I find that people like us are often encouraged to apply to colleges based off of nonsense like state school numbers, but this ignores that the state school students at Cambridge will usually have more in common with the public school students at Cambridge than with us, and that by being steered away from the 'posher', richer, older colleges, we're also being steered away from having our financial burdens alleviated as much as possible while at university.