Well, Material Design is just a set of guidelines based on some research. It suggests good practices and consistent solutions. Material Design guidelines used to be pretty strict, for example when it comes to shapes of buttons, because they wanted to make users used to some concepts. With Material Design 2 the guidelines relaxed a lot.
Minimal size is meant to keep items distinguishable and accessible, but still not too big. I wouldn't make tabs much smaller, because it may look squashed. It won't affect the usability though as minimal size for clickable items is 40dp, I guess. You would probably need to reduce spacings and sizes of other items in your app in order to make it look consistent with shorter tabs.
Changing font is fine as long as it was designed for on-screen reading. The size can be any as well as long as text is large enough to be readable.
If your design is consistent, items are distinguishable, easy to use and aligned well, then you're good. Take a look at Material Studies on material.io - these designs show how far you can go without breaking anything.