I don't consider feat taxing to be a viable compromise. I'd be better off just taking a single spell per form. Also, if one took the Monster Blooded Feat, one would simply take levels in that class. They would also have to already BE a monster race (if I remember correctly, anyhow; they were "Monstrous" feats, right?).
There are more extreme/dangerous measures to consider.
Option 4: Permanent?!
As a caster utilizes the spell, they find themselves drawn more and more to its effects. Whenever a caster assumes a monster race form via a polymorph spell, he adds an addiction point to his sheet. Every time a caster utilizes a polymorph spell and has an addiction point, they must make a Will save upon transforming (DC 10 + Spell Level + Addiction points) or they gain the personality and mental ability scores of an average member of the race class, including its natural alignment. These addiction points do not go away naturally and can only be removed via counseling (hire an NPC) and a Wish spell to remove 5 points.
Option 5: I did WHO to WHAT now?!
Casters don't just gain the form of what they turn into... they channel its very essence! Whenever the caster polymorphs into a race class, they must make a Will Save (DC 10 + Caster Level + Casting Mod) or become, for all intents and purposes (as Option 4), a member of that race class for the duration of the spell and are unable to dismiss it. They lose all memory of any events that transpire during the shape change and are under control of the DM until they revert back.
Option 6: Put your left foot in, take your right leg ou--owwwww!!
Sometimes physically manifesting yourself isn't as painless as one would like. The caster must make a Fortitude save (DC 10 + biggest different in physical stat after racial adjustment) or take a number of points of damage equal to the difference times the spell level (ex: STR 10 wizard to STR 25 Tarrasque --> Fort 25 failed --> 15 * 4 [Polymorph spell] = 60 damage).
And again, there are any combination of options available. And more options to consider, I'm sure.