Could He Eat Rat Pups?

Could feed him fuzzies, just up the number/frequency until they're all gone. If you get 10, then he'll be ready for pups by the time those run out.

Ordering 25 pups and 10 fuzzies. Will my 166g be ready for the next step when the pups run out?

The 166g can take pups now, they're within the 10-15% guidelines. When the pups run out, he should be, it depends on how fast your normal ends up on them and how often your g-stripe eats. That young they'll probably both eat every time. Weans run 30+ grams, he might even be able to handle smalls (40+) if we're assuming Layne Labs here.