I want to make an Horse Emblem!
I've got Reinhardt.
I've got a +Atk Cecilia.
I've got all the mounted GHB heroes + I've got a shit tons of feathers and skills to inherit.
Is Reinhardt \ Cecilia \ Xander the most robust core? How to distribute the horse buffs between them? What positioning skills should I give each one? What about other skills? I want to break the game
Camus is a good fourth for that.
My own horse team is Xander, Cecilia, Camus, Reinhardt in that order. Xander runs Fortify Cav since he needs it the least and Camus runs Hone Cav as he hits decently hard as is. As for Support abilities: Got Swap? Everyone wants Swap in a horse team unless they have something similar already.
Now, the gimmick here is Cecilia with Gronnblade. Got a Nino? Steal her book for Cecilia. Oh, and grab Drag Back off her while you're transferring the Gronnblade. Then laugh as the combined buffs of Fortify and Hone Cavalry turns into +30 Atk thanks to the Blade.
Goad Cavalry on Reinhardt and Cecilia is also fun, for even more boost when grouped up. They're not
as critical, but very nice to have. And Goad stacks with itself, but doesn't boost Blade tomes.
For builds on the different units, there are several that work. Xander can run Fury/Vantage with this, but a more defensive build is also an option. Camus is the same, I build mine with a Quick Riposte build. Reinhardt and Cecilia want Death Blow and if you don't have that, something that boosts Attack. Cecilia's own Attack +3 is fine. You're already in gross overkill territory most of the time with her.
Now about Reinhardt since you did say you wanted to
break the game. Moonbow + Quickened Pulse Seal. Reinhardt will trigger Moonbow on the second hit of his attack in his first battle. Give him Lancebreaker, because what else would you give him, and he's done. Most evil build in the game before horse buffs. After... Yeah, there's very few units that'll survive that.