Introducing young roo to mature hens

[112Days]

maybe but that wouldn't be good genetically as frizzle x frizzle breedings create frazzled chicks which are genetically weaker and have brittle feathers and i don't want to risk that so ill have to breed them to a smooth feathered roo

Not entirely true. In a nutshell, frizzles are actually heterozygous for the frizzled feathers trait. Frazzles are homozygous for the trait and are the weaker birds you describe. Using a simple Punnett square, the chance of a frazzle from a frizzle x frizzle breeding is 25%, though they tend to self-terminate. There's a 75% of a healthy chick - 25% chance for a smooth baby, 50% for a heterozygous frizzle.