To attack a territory, I understand that I need to be adjacent to it. But let's assume this, I am attacking Colorado Springs. All territories surrounding it are owned by me and are occupied. Here lies the question. Can I attack Colorado Springs from multiple territories and use units from multiple territories? If so, can I use more than 5 units to attack Colorado Springs as long as only 5 units or less actually invade Colorado Springs?

Yep, that's the way to smash your way in, and remember you can stack 5 bombers over Colorado Springs as well if you are really feeling paranoid with your dice.