Is it safe to feed my rats frozen veggies?

Yes that's fine as long as they are cooked properly and have cooled down, remember greens are to be restricted to every few days and in small amounts or can cause the runs.

Frozen food these days is nearly as good as fresh because the producers can freeze it in a matter of minutes now, old frozen food was bad because there wasn't the same technology as now, so the food would perish slightly while it was in the process of being frozen over several hours.

It's often the case that people who think frozen food is bad probably tried it in the 1980's and never touched it again since.

EDIT: Cook it from frozen as per instructions, let it cool down and then serve.