How to breed lots of Roostros?
Solution 1:
AFAIK, you can't force a Chickadoo to become a Roostro. The best way to get a lot of Roostros is simply to improve your coops as much as possible to breed more chickens. Mochi Miles can provide you with special coop upgrades when you visit her Manor and gather enough Quicksilver Plorts for her.
Viktor Humphries will also provide you with the blueprint for a gadget called the Chicken Cloner. If you shoot a chicken into the cloner, it has equal chances of either coming out as two chickens or disappear completely. This is in fact a crappy, but when it comes to getting more Roostros, it might help you get started.
Alternatively, you could remove the need for meat by combining the carnivore slime types you keep with herbivore slime types, and feeding them the other type's favourite food instead. Feeding a largo either of its favourite foods will always give double plorts of both types, so if you choose your combinations properly, you can set up your corrals so that you never have to feed your slimes meat.