Are you talking about the fittings for the bridge?
I recently put my 2 GTX 580's onto a bridge and had to go elsewhere for the connections. For some reason OCUK only do blanks which is basically just to fill the space of where a graphics card would be if you don't have that many, like for example if you had a 3 card bridge and only 2 cards you would use the blank on the 'empty' slot to stop the coolant flowing out basically.
So yea you need to pick whatever your bridge itself is. If im correct the actual fittings are simply made for the card type, it is the bridge itself that decides whether its a serial or parallel setting.
If your unsure too a parallel is meant to cool both cards at the same time whereas a serial will pass the fluid through the first card and then onto the second. The idea is that a parallel 'should' have the same temp on all cards connected to the bridge whereas the serial connection would have any card after the first that is cooled at slightly higher temps due to the fluid heating up as it passes through each card.
In reality it makes no difference as far as I can tell. I have 2 cards on a parallel bridge and even now with me just browsing I have card 1 at 36 degrees and card 2 at 34. It isn't rare for the temps to range further on load too!
Hope this helps.