Before the interview they may just not respond. Most PIs just tend to be very busy and if they're not on the admissions committee they aren't gonna say much pre-interview. Post-interview (really post acceptance), they may be more inclined to answer your questions. This is all based on my experiences so others may have other things to say.
I didn't really see a difference in approaching faculty at various sized institutes. Just be polite and concise. Now if the PI happens to be really high up the academic food chain (think academic administration), they may be a little hard to reach so reaching out to the program coordinator, secretary, etc. may be a bit easier.