Anti-multi-action

No, bigger vampires without fortitude remain as limited as they are now and get no better while the others take a step down.

No, the only thing that will take a step down is fatties playing 20+ Freak Drives. Which isn't neccesarily a bad thing. In fact I'd argue it'd be a good thing.

Normal Freak Drive use remains unchanged.

If punishing multi-acting is your goal, maybe Vampiric Disease is really what you should play more.

Master.
Put a disease counter on any vampire. Each time a vampire with a disease counter is in combat at close range with another vampire, the second vampire gets a counter as well. An afflicted vampire cannot gain blood by hunting. When an afflicted vampire untaps, he or she burns a blood or, if unable, burns the disease counter. A vampire can have only one disease counter.

totakly agree with miasmat, vampiric disease will shut down an Una multi action deck anytime.