Thanks for the answer!
Would the refresh discount be limited by the number of form points that can be dedicated to Toughness powers or by the refresh cost of the Modular Abilities power?
Not sure I understand the question but A toughness power has to always cost 1 refresh.
So, if you have 2 points to assign in modular abilities, and you take inhuman toughness, with a catch of +3, toughness is still going to cost 1 refresh - despite the rebate. But it means you might as well take supernatural toughness instead (or an inhuman toughness recovery combo)
So your character might look like this:
-6 modular abilities (-2 base)
-4 form points
Bear form:
-4 Supernatural toughness
+3 catch: Weapon 3 weapons or higher.
-2 inhuman strength
-1 Echoes of the beast
Edit: you should be careful about taking multiple catches. Check with your GM. Being able to switch forms and catches might be too powerful since you could switch forms if someone finds your catch mid-combat. Generally, you take a universal catch, I think.