They have to be the same model in order to run in sli. You need either 2 550tis or 2 560tis
I read that you can use a 560 and 550 together if you downclockhttp://www.tomshardware.com/forum/362676-33-550ti-560ti
From the thread you linked to :-
Wait.....I think everyone in this thread is telling you that you can run a GTX 560 Ti and a GTX 550 Ti in Sli together......This is WRONG. To run cards in Sli they must have the same GPU. Sli is not like Crossfire where you can mix 2 different GPUs.
You can only Sli a GTX 560 Ti with another GTX 560 Ti or a GTX 550 Ti with another GTX 550 Ti.
You can mix graphics card brands. So an EVGA GTX 680 will Sli with an Asus GTX 680 ect.