I usually add more strawberry base, a little more milk, and one more scoop of protein.

I guess you can do it without? techincally?? but it'd be watered down, not fill the whole cup, and other things that just make it super sad and not worth the price so they didn't want you disappointed? :(

but i would think that you already know all this and expect it since you're a barista so maybe that one barista who helped you was just strict ?

When that happens at my store I suggest a Strawberries and cream frappe with protein added.

I made myself one today without banana by accident (brain fart) and it came up a little short in the cup but tasted like cereal! I couldn't put my finger on which one (Maybe Fruit Loops?) but it was really good!

I don't see why not having banana would be a problem as long as you were understanding it would taste different.

We sub a package of blueberries from the oatmeal when we're out of bananas if the customer really wants a smoothie

You can.