Your questions are very good ones but are often difficult to answer within the confines of a one day, structured visit to a given department. Ultimately, a rotation is the best way to answer these types of questions. Anyway, that's not what you asked, so here goes:
That is, do resident and faculty staff need to really get along (both within the group and between the groups)?
1. Resident-Resident interactions: IMO, this is of critical importance. Especially in the beginning of your residency you will need to lean on the senior residents for support. You will need to ask them how to resolve various patient care issues, dosimetry issues, on-call issues, etc. Later you will rely on them for their job interview experience. Residents who hang out together and/or get along extremely well is a mark of a healthy program.
2. Faculty-Resident interactions: During interview day, the residents are more likely to give you the truth than faculty on this matter. In my experience, this boils down more to how individual faculty interact with residents. Younger faculty tend work better with residents and ususally teach more. Given a large enough department there are always a few bad apples but as long as most faculty and residents get along then that's optimal.
3. Faculty-Faculty interactions: Very difficult subject to broach in an interview setting. Even if you rotate at a program for one month, this is hard to ascertain. As a (not so new) PGY-2, I am slowly learning more and more about the subtle political relationships within my department. As long as there are no obviously negative issues apparent, you should not have to worry about this much.
What if the program is really good but kinda toxic?
Avoid at all costs.
Seriously.
If you don't feel comfortable not ranking a program, then move it to the bottom of your list. Rad Onc is a long residency and you will be spending a lot of face time with faculty. This medical speciality is like an apprenticeship and if you don't have a good relationship with the "master" then you will be absolutely miserable. Also, Rad Onc is a field that by its very nature attracts easy going personalities. If you can tell a program is toxic from a single interview day then it must be very bad indeed.
Each year, as reported by applicants, Rad Onc residencies can often change in subtle and not-so-subtle ways.
Take the following examples:
1. Duke: Had a history of being highly malignant; from recent reports, this seems to have turned around completely
2. Jefferson: History of disgruntled residents; this has appeared stable over the last couple of years; though since Curran is leaving for Emory who knows what it was like this year?
This is why it is very important for applicants to review programs candidly after interview season is over.