I am looking at pedals that are compatible with Shimano sh51 cleats only.
I don't think there is any, if the SH51 fits then the SH55/SH56 will also fit.
What is the difference between the other shimano cleats I see for MTB pedals? IE the sh55 56 etc...
SH51 = single release, they only release when the heel is twisted outwards
SH55/SH56 = multiple release, they release when the heel is twisted in any direction
SH55 is no longer made, it was replaced by the SH56.
http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Pedals/SI_46F0A_002/SI_46F0A/SI-46F0A-001-ENG_v1_m56577569830701980.pdf
You can see the specs and photos of the different pedals on the following link.
http://productinfo.shimano.com/specifications.html
MTB / TREKKING > Pedal (SPD) & Pedal (Click'R) > Submit
When the results come up, click on the pedal Model No. to see the pedal.