Surround back spkrs in 7.1 vs 5.1 surround spkrs

In a 7.1 setup....if a 5.1 movie soundtrack is being played, do all seven speakers engage or just 5? In this instance, does the sound just go to the surrounds rather than the surround back speakers?

In the converse...if a 7.1 soundtrack is being played through a 5.1 system, does the surround back tracks get played through the surrounds? In other words, does all the information get played back in two separate rear channels rather than four surround channels?
as it was explained to me, in the 7.1, all 7 engage but if only 5.1, your processor duplicates the surrounds onto the back speakers---if you have a 7.1 soundtrack( good luck finding one) and play on 5.1 it'll eliminate the back tracks and only play the surrounds,center, and front tracks
The 6th and 7th channel are a matrix of info from the 5.1 surrounds, in time there will be true 7.1 content but not just yet. If you are talking about true 7.1 material then its the decoder for either your DVD or movie Pro/Reciever will control the steering based on your speaker configuration so your not simply missing sound because of fewer speakers.
I'm currently running 5.1. I have a huge room and I want to add another pair of surrounds...either as back surrounds for 7.1, or running four surrounds in 5.1 via "Y"-connectors. Maximum coverage for the whole room is what I'm after.

I have the capability to go 7.1, but I want all 7 speakers engaged for every movie that I play.

My dvd player decodes all signals, Hi-Def and regular through the analog outs.

So, according to the explantions above, if I set up a 7.1, and play a 5.1 disc...the back surround speakers will engage. So, if I set up a 7.1, all speakers will engage for either a 5.1 or a 7.1 track.

I already have all of the equipment to do this...I just wanted to make sure what my options were before I installed the second set of surrounds.
Yes they will but they are the least active speakers in the chain just offering a bit of fill so that may or may not be enough for your taste in a big room like you have.
If I were you I would and a second pair of middle surrounds with a splitter if you have to and set them up maybe 15ft behind first set and still add the rear surrounds for most impact, use di-pole for sides to maximize dispersion.
I just finished installing a pair of new but discontinued matching speakers to my 5.1 HT room in anticipation of a new display, 7.1 receiver, and player next week. Currently I'm driving them with an Integra 5.3 receiver which accommodates L, C, R, Surrounds, and one rear.

With the old smaller surrounds now doing duty at the rear, a channel I hadn't been using. The room has taken on a new fuller character when used in conjunction with an Integra DSP mode for standard broadcast. The rear channel allowed me to relocate the surrounds slightly more forward which enhanced the sense of front to back with DVD playback in my room. IMO it was definitely worth the effort.