My room is 12' x 20' and all the speakers I mentioned previously work well in it set up on the short wall about 4 ft. into the room. If you intend to put the speakers in your home theater room you could probably get bigger Maggies and still do just fine.
As for your subs using them with the Maggies would be fine. However, I'm a little confused as to the position of them in the room. What I think I understand is you have 4 subs total, two that are positioned somewhere to the left of the listening position and two somewhere to the right. Perhaps you can be more specific as to their positions. I assume you want to leave them in place for the HT set up. Hopefully the addition of the new speakers will allow that to happen.
As for your subs using them with the Maggies would be fine. However, I'm a little confused as to the position of them in the room. What I think I understand is you have 4 subs total, two that are positioned somewhere to the left of the listening position and two somewhere to the right. Perhaps you can be more specific as to their positions. I assume you want to leave them in place for the HT set up. Hopefully the addition of the new speakers will allow that to happen.