Well, assuming you want to turn into a (more or less regular) rat, then your powers might look like:
-1 echoes of the beast
Beast sense: enhanced sense of smell?
Beast trapping: eat anything?
-1 beast change (this allows you to change your skills among other things, you are probably stealthier as a rat, for example)
Human form (+1) covering:
-1 diminutive size
-2 inhuman speed
-1 cloak of shadows?
For a total of -5 refresh
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Assuming you want to go for a sorta rat-man form instead, you might have:
-1 echoes of the beast
Beast sense: enhanced sense of smell?
Beast trapping: eat anything?
-1 beast change (this allows you to change your skills among other things, you are probably better at fighting in rat-man form)
-2 inhuman recovery (you heal quickly, even in your human form)
+2 the catch (silver: obtainable +1, researchable +1)
you can't zero out toughness powers with a catch; you can have a +2 catch because you also have inhuman toughness, another -2 points in you beast form. If you didn't have inhuman toughness as well, this would only be worth +1
Human form (+1) covering:
-1 claws
-2 inhuman speed
-2 inhuman toughness
For a total of -6 refresh