yes, set basic to 6. because if you set it to 54 then MT will not connect at all if the signal quality is too low for this kind of speed. however if you set it to 6 then it will automatically choose at which speed it can work, even 54
When doing a MT PtP link, I usually lock both Basic and Supported to the second highest speed that is stable (i.e. if the link works well at 54, I set Basic and Supported to 48). Locking the speeds down does seem to have a positive effect on jitter, though it does make the link somewhat inflexable.
If you do decide you want to lock things down, be sure that you set “rate-set=configured”, or it will ignore your custom settings.