The option to use a 50*50 mm base Hippogriff with Damsel is not a legacy issue because it doesn't exist, so that would not resolve concerns about the model's base size. Hippogriffs we're never intended for Damsel's.
My mistake, I wasn't very clear. I mean if the option were available in the LAB for a smaller Hippogriff without Relentless, you could take -
A Lord on (small) Hippogriff + a Damsel with Divination
and then keep giving the attribute to the Lord.
More important, the community should consider if it would rather keep 50*50 and forfeit the new rule buff as well as Towering Presence. Remember the mount would receive a rule to prevent joining units.
- that got me wondering whether both is potentially an option.