Not sure if it's offered in a 550, but I "believe" AdamZ, Chevy, Bekit, and p3Droid (not sure but that's close) use kernels that are 600 and still have tether.
Just have to find their pages and descriptions to verify.
If not, you can OC to 1000 and use SetCPU to down clock to 600 - just check how many slots and the slot speeds to ensure at the very least a 600 is there.
Edit - DL ROM Manager. Go to settings and enable advanced mode - go to Download ROM - click CyanogenMod at the top - click P3Droid Kernels - Click the first kernel (600) - it will prompt the whole install, just choose the backup option first and no wipe.
Just verify that this one has tether (most do)