I have read great things about cross breeds then other people say you should keep purebreeding.
The one thing to keep in mind with this regard is the goal. With our chickens we are raising exhibition stock so there is no crossbreeding, with our rabbits we are using them for the table so we cross. In most production scenarios there is an advantage to x-breeding (hybrid vigor). I think where most people falter in cross breeding is it not a thought out process, and you end up with many breeds in the background going in many directions and then you loose the advantage. Either way you will need quality purebreds from one of two sources: raise yourself or have replacements from a reliable breeder.
for example:
Three breed rotation
1.♂ breed A x ♀Breed B = Offspring 50%A50%B
2.♂ breed C x ♀AB = Offspring 25% A25%B50%C
3.♂ breed A x ♀25%A25%B50%C = Offspring 62.5%A12.5%B25%C
4.♂ breed B x ♀62.5%A12.5%B25%C = Offspring 31.25%A56.25%B12.5%
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5.Breed back to Breed C and equilibrium is maintained
However this can get complicated, but just like any breeding system when done right records are important. For us right now our buck is the crossbred and the doe is pure . and depending on what where we are in the system (and what we have had for offspring) start over on the rotation, since we only keep a 1 buck and a couple of does. But again it comes down to a plan and goal.