With the addition of Hunter Slimes, I find myself needing a lot more Roostros than I ever have, but Chickadoos grow into them much more rarely than they grow into Hen Hens. Are there any ways to make Chickadoos grow into Roostros instead?
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.
I don't think there is a way to really breed Roostros more effectively. There isn't a 100% chance that a Roostro will make another Roostro in it's lifetime. I'd recommend 2-3 roostros in a coop with 3-4 more hens of your choice. You should definitely trade with Mochi Miles so she invites you to her Ranch. You can then get a better coop and hold double the amount of hens if you take up the deal she offers you on her ranch.
My favorite way of getti g roostoro is finding a bunch in the wild and throwing them into a chicken cloner. It wont clone it everytime and theres even a chance you wont get anything out of it but it clones most of the time. To get the chicken cloner recipe you need to complete viktor humphries tasks in the simulation.