Hi!
What you are suggesting is fine, you can use the stoichiometric coefficients in an elementary reaction to find the exponents for the rate expression, and these exponents will represent the order with respect to each component. Page 98 of the classroom companion shows an example of this.
In any other type of reaction (if the MCAT doesn't explicitly state that the step is elementary) you should not assume that you can use this method, and you would need to use experimental data to find the order with respect to each component.
I hope that's clear!
-Laura