Multiple Companions

It is actually possible to get all of the companions at once via glitching.

See here for details.

Yes, I've looked at that. But none of the circumstances listed seem to meet what happened to me.

Check out the entry for Star Paladin Cross.  Supposedly they fixed the fact that she would join you regardless of number of current companions, and without having to fire them first, but it would seem that is not the case.

yeah, I thought about that. I've fired her and re-hired her several times though, despite the fact that I have Fawkes with me.

Try firing Fawkes and taking on some other human companion, then see if you get the same results.

The only other companion I could get is Charon. My karma is too high for anyone else.

What I meant before was that if they didn't fix it, Star Paladin Cross will always join you if your karma is high enough, without any regard to whether or not you already have other companions.  If you get the same results with Charon (or anybody else that you haven't tried yet) and Cross that you get with Fawkes and Cross then that would be evidence that they didn't actually fix the bug.

Well, my other character who also has Very Good karma asked her to join and she refused, and that was with Fawkes. She said something along the lines of "I cannot so long as one who is not of the Brotherhood travels with you."

You have to use the  'I don't suppose I can stop you' response to get her to join when there is already a companion.  Is it possible that with this character you used that response and you did something else with the other?  If not, then the only other thing that I can think of is that with one character you played with the most recent patch and with the other you played without it.  Other than those two options I don't know of any other way this could have come about, though with the number of bugs and glitches in the game it is always possible (probable) that there is another one out there that I'm not familiar with.

when i played FNV i had the dog - just to carry extra stuff - but now i've gone back to F3 i don't use companions - i find it more enjoyable to just be the lone traveller guy...