Hi Lilia,
I'd say sidney is a great choice for picking the MML course, as you get a lot of support from your college DoS and tutor with regards to guidance and extra help you might need - there have been numerous times when I've requested extra supervisions (for revision etc) on certain topics which the department didn't provide, and college were very willing to let me have those. Sidney has one DoS for people doing 1st and 2nd year, and another one for those on their year abroad and doing finals. Both are lovely and really friendly and approachable, and won't intimidate you at all.
In terms of atmosphere/tutors etc, a lot of stuff with MML is organised through the faculty, so it's not necessairily down to college choice whether you get certain supervisors etc. We do have a german Lektorin and a French Lectrice, so if you're studying either of those you will be supervised, as least for oral and some translation work, by them. Language classes (for all langs) are organised centrally, so you'll have little choice over who you get. Most people are nice though (so I've heard?!)
Let me know what you're thinking of studying - bigger subjects like french, german, russian etc have more college organisation than say, italian, which is completely faculty based, so specifics info will probably give you a better idea of what it's like. In terms of atmosphere, sidney generally take between 6-8 modern linguists (though this can vary) so we're a fairly sizeable bunch in a small college!