It's better to breed up. You get a better feel for the Pokemon that way and what works well or new tactics that not everyone is using.
I can normally tell a hacked poké. They're typically competition level and crappier than I am at battling. They're also normally popular pokes and half the time they're shiny.
You can get a comp Pokemon with an hour and a half of breeding and an hour and a half of leveling/training that'll do you far better.
But yes, as long as they're 100% legit. Ribbons, met location, etc. you can use them.
(Don't egg move any caught pokes. It won't work. Only egg pokes.)