Hi Rodjeur,
From my feedback, to configure interconnexion between VOSS and other vendor device, i always use LACP (MLT or SMLT).
If it's not required, i always disable spanning-tree on MLT/SMLT. And especially with other vendors, to avoid STP compatibility issue.
SLPP is a good way to check SMLT static link agregation, i use it too.
With LACP, some says that as LACP check the agregation and it's not needed. That's true, but SLPP add another protection.
Regards,
Théo