1)
Using only four speakers, and no center, will work just fine, and won't cause any issues.
As long as you set CENTER=NONE, dialog that would normally come from the center channel will simply be routed to the left and right fronts.
(However, many people find that adding a center channel speaker helps "pin the dialog where it belongs in the center".)
2)
Yes and no.
The T1's actually go pretty low on their own.... but they don't go quite as low as a sub (and they can't play really low notes quite as loudly as a sub can without distortion).
Surround sound movie discs actually have a separate channel for low frequency effects (actually called the LFE - Low Frequency Effects - channel.)
If you have a sub, the LFE channel will be sent to the sub.
And, if you set your other speakers all to LARGE, your other speakers will remain full range.
(And, if you have no sub, and set SUB=NONE, then the LFE channel will be split up and diverted to all of your other speakers.)
However, if you have a sub, and you set some of your other channels to SMALL, then frequencies above the crossover frequency will continue to go to the speaker, while frequencies below it will be diverted to your sub.
By taking some of the load of producing really low bass from the T1's, they will be able to play a little louder, and the amp that powers them will need less power (so it will be able to play a bit louder before it distorts.)
3)
BY DEFINITION, if you have five speakers, then you have front left/right/center and l/r surrounds.
You can have surrounds but not rear surrounds; you CANNOT have rear surrounds but not side surrounds.
(So, if you have five speakers, even if they're physically in the back of the room, your two extra speakers will be "surrounds" and NOT "rear surrounds".)
The answer to your question really depends on how big your room is, whether you listen to more music or more movies, and how much you like bass.
If your room is relatively small, then the difference between 5.0 and 7.0 won't be that noticeable.
And, if you listen to mostly music, and don't just love huge amounts of bass, then a sub may not be all that noticeable either.
HOWEVER, if you listen to a lot of movies, or like a lot of bass, adding a sub will probably make more of a difference than adding another pair of surrounds.
I ended up ordering the T1s.
At the moment, I’ll use those as R/L and no center or sub. That will be in the future.
Would it be weird to have a front L/R and then a rear L/R? I do have two other speakers to do that with. More that weird, would that temp set up cause any sound issues?
When I do get a sub, does that essentially negate the woofers on the towers or (as I suspect) is this what is meant by crossover? That at some point the woofers on the towers stop and the sub takes over? I may have that all wrong and probably do.
Finally. When I get a center (my next purchase) I will have a front L/C/R and a rear L/R. With the T1s as L/R, would i most notice adding two side speakers for essentially a 7.0 (assuming the bass on the T1s is fairly good) or would I more notice the sub and go for 5.1 before adding the sides?