Most insurances will charge you a "penalty" though they wont call it that. it will be disguised with wording to make it sound like its doing you a favor.. like patient advantage or some other corporate speak..essentially they will charge you the difference between the brand name med and the generic plus your copay. ie; brand total cost is 800 generic total cost is 100.. youll be responsible for 800-100=700 + copay.
Pretty sure 99% that if a generic is available it WILL be the preferred medicine on the formulary and you WILL be charges a non formulary cost unless your insurance is generous to allow prior authorizations or patient level authorizations to make it formulary for you.
In regards to generics, the FDA mandates that the generic does the same thing clinically as the brand, and must contain the same active ingredient (though a variance of 20% difference is allowable), the only thing that is allowed to be different is the inactive ingredients (dye/color, filler, etc)