I did some tests with the 60x60mm regiment tray and the chariot.
The horses are smaller than the ones I will use for the rohirrim, maybe I will use some alternatives from zvezda or I could just consider them as a race of small horses used for chariot.
I ordered the 60x60mm regiment trays with slots, here how they should be done, the first for the infantry, the second for cavalry and the last for the chariots.
Then I could easily change miniatures inside the regiment trays when needed, and use them as single miniatures if I want to play another game with skirmish rules: