Well the tradition with monarchs back in the UK, and also continued to Canada OTL, was simply to have an image of the monarch sitting in state or some otherwise regal position (the current seal has QE2 on horseback I believe) so I'd say there's every reason to just fall back on tradition here.
As for the supporters (what you called "holders") of the coat of arms, puma and alligator seems fair but if you really want to test the skill of the person who's going to be making this stuff for you then there's plenty of room for added detail in the CoA. CoA could be full of allegory and symbolism. For instance, might I suggest that the Puma be the sinister supporter (supporter on the left) and the Alligator be the dexter, to represent the west-east span of the country reaching from sea to shining sea? You might also want to have the Puma hold a broken chain, to represent the end of slavery and the defeat of the Slaver Uprising. If you really wanted to go overboard then you could have the puma be looking up and raising a shield while the alligator looks downwards (it's probably more natural for the alligator to be lying down with its tail supporting anyway, rather than standing on two legs looking like a man in fancy dress), to represent the state guarding the people from the aggression of their northern neighbour, while keeping a beady eye on their less stable southern neighbours, but that might be going overboard.
Break out the Kentucky Bourbon or Virginia Wine 
