Yet another question...behavior ?

The larger guinea pig (the one that I think might be a male) always rubs up against the other guinea pig and kind of sways back and forth (like as if he was going from one back foot onto the other and just keeps repeating this). And while she is doing this she also is making that purring/rumbling sound.

Why do you think she/he is doing this? Does it sound like a mating thing or does it sound like she/he is showing dominance? Any other reason?

I think what you are describing is the "rumblestrut" - I believe it's a dominance behavior. My pigs do it occasionally and it seems to me that one is trying to say to the other "don't touch - no monkey business here!".