I was just wondering cos it says that neutral evil only play for self benifit and would betray their comrades to 'upper' themselves etc. So does that mean that a neutral evil could do good and be rightoues if they knew that they would almost certainly benifit from it?

of course you can do good, it just wouldnt be aligned to your pc's character background. you could consider changing it if you really wanted your character to (ie. ShadowKeeper) but, otherwise it wont effect your game drastically.

I normally play a neutral evil fighter. I like to think of him as a mercenary he'll help people out if theres some money in it. Therefore IMO neutral evil can do good but only if they are thinking about the money/rewards.

Would a neutral evil character want to have companions with the same alignment as him or would he want other kinds of alignment?

If they are useful to him, and won't kill him, anything goes. Just don't do a lot of torture and childkilling with any goody-goody types around, they might get upset.

I guess that someone of one alignment might like to take someone with them of the opposite alignment to manipulate them, convert them to their side, and stop them from doing things which would thwart their own plans.