Does the rosary have to be said a certain way to be effective?
The short answer
When praying privately, you can mix and match. The object of the rosary is prayer; meditating on one mystery for all five decades is fine and not disrespectful.
Private prayer
Praying the rosary privately, you can mix and match.
The point is prayer
If you want to pray five decades meditating on a single mystery, that's fine; there is nothing disrespectful in it.