a female rat can produce a litter once every 20-24 days. each litter can be anywhere between 8-18. It's best to get young female rats to start, instead of full adults (if too old they won't breed much or at all). Typically 8-12 weeks old is the best age to get the females. Males can be older or younger.
If you know you can sell off excess babies then I would go with two colonies of 1:2 or 1:3. You could have just one male and move him back and forth in the colonies or have two males to save the trouble. As new colonies can take up to 6 weeks or more to get settled in and breed, it will be hard to stagger the first litters. If your snakes can eat frozen thawed, t hen you can freeze extra of the sizes you need so they are useable when between litters.
Rats wean at about 3-4 weeks so you can separate some into another container to grow up more for the larger sizes you need. If you find you need more later on, you can raise up daughters to add to the breeding colony.