It'll likely take awhile for the girl to be ready for mediums, so here's what I'd do: As long as the rats you have are appropriately sized to the snake's girth, then you'd be fine feeding off the mediums to the Firebee once/week until they are gone (then switching back to smalls permanently for him) and feeding the Pastel smalls once/week (until she can move up to mediums).
You don't want to end up with the football that was mentioned above - just a slightly noticeable bump that lasts for a few (2-4) days at most.