Not sure about the first question, but what you're describing in the second one is called rumble strutting! You may also notice them sway their little butts and do nose offs (where one tries to get his nose higher in the air). It's a display of dominance. Since they're babies, they're probably just trying to figure out which one is "in charge".
My three boys do this all the time and they're not litter mates to my knowledge, but they have been together since they were at least 6 weeks old. We've never had any fights, thankfully, but if you do have a fight on your hands
never put your hands in between them! It's best to throw a hand towel on top of two fighting pigs, as it seems to confuse them long enough for you to scoop one up.
Do check the Cavy Spirit pages, they are very informative. There's a bit about safe dominance displays and displays with the intent to harm towards the bottom.
http://www.cavyspirit.com/sociallife.htm
If you haven't heard of Guinea Lynx, be sure to bookmark it too. I trust these two resources more than any other!
Guinea Lynx :: A Medical and Care Guide for Your Guinea Pig