I don't think you have to worry at all about changing the airfoil by sanding the ribs level on the bottom The larger issue is whether the attachment joints with the TE have been weakened by reducing the effective contact area. Seems to me that if you sand something off the bottom you'll need to add a bit of balsa to the top rear of the ribs and sand to shape and this could be a delicate operation.

But then I don't know anything about this kit. Is the TE notched to receive the ribs? If so, this is probably not a problem.

I think the largest danger for first-time builders is not using the right glues in the right places. The most likely disasters will be bits of the model coming off in flight! Second would be not spending time to making sure the wing is properly aligned with the elevator.