You wont loose volume.
Yes different distances for different speakers is normal. The distance setting & individual speaker adjustments will make up for those things. If setup correctly it will sound great.
These are very novice questions which is ok but id recommend reading up on Home Theater basics 101, speaker placement, speaker setup etc. Here on Audioholics you can find great explanations on all of these topics.
If you want a great home theater experience then yes, adding a center channel & 2-4 surround channels, will enhance your movie watching tremendously!

Dont forget to add a good sub!!! Thats VERY IMPORTANT!! Even if you think your main speakers have bass, its not enough for a good movie theater feel. That reminds me to tell you to read up on Proper Bass Management as well. Trust me, a little research on your part before jumping into this will go a LONG way & save you some headaches!!
I hope this helped you!!
BTW... Welcome!!