Key is that all components of a mix are in the same size bracket. Your pumice is great for medium and large bonsai pots, and grow out containers, meaning training pots. Nothing wrong with the pumice, great for training pots for young pines and other species. Do not mix 1/8 - 1/4 media with 1/4 to 1/2 media. It will make a more dense mix with significantly less air voids than either fraction by itself.
I would use the pumice for 3 and 5 gallon size nursery or grow out containers for training younger or less refined trees. Good also for large bonsai pots, pots over 12 inches in one or more dimensions.