Absolutely NO wrestling, roughhousing inside, zip, zero, nada, ever. Inside is Switzerland and rules must be honored.
Lots of separate training and supervised outdoor play.
Puppy must earn all his/her toys, food and play.
Reinforce Ragnar’s rules and training.
Let the puppy watch you work Ragnar while in his crate outside. Puppy will most likely pitch a hissy fit wanting to join but that will be good distraction training for Ragnar.
Do not allow them to tag team the cats.
One on one, my twins are perfect with the cats, together they become hellfire missiles.

Keep them separated when you are not around by a baby gate or puppy in crate. Toys are iffy, I would not allow them to play with toys together until pack order is established. Puppy will have to learn respect for Ragnar without being terrified by him.
I never allow mine together unsupervised as they love to wrestle and tussle and a lot of times it escalates to me having to intervene.
Get lots of rest (you will definitely need it lol). If you get frustrated, just back up and regroup. I call it my; “Plan your dive and dive your plan.” Like we did when scuba diving. If something is not going as planned, ditch it and come up with a different plan.
If I understand correctly Ragnar has an edge to him so you may want to consider the temperament of the female to help you quell a challenge later on.
My two had their period of crate and rotate because of snitting with each other.
I had to up my game as their leader to get a handle on it because crate and rotate is very difficult and I wasn’t going to live like that.
Your puppy should learn a lot from Ragnar with all the training you have done with him. Make them both toe the line.
Wear a ball cap so you don’t pull your hair out and know that every time you question your sanity for getting another ‘Doberman’, they grow up fast and furious, but the joy, love and respect lasts beyond a lifetime!
Looking forward to your plans of adding another to the pack.
