Wormtail's a rat but he's not exactly obedient. Just scared shitless of Voldemort.

Still...

"A rat. A waitrat serving tea and crumpets all day. It could work. And he would be harmless and helpless to boot."

In other words, it could work.

"But how do we turn him into a rat? Neither of us could do it, least of all myself, considering I'd have to be the bait needed to get him where we want him to be."