My thinking is you'd want some large calibre pieces and small calibre pieces so they can be bought together when fitting out your ships.
You might have to double up on gatling guns etc, you really don't have much room to manouevre, its either gatlings or vickers or something and they're all more or less covered, but I suppose you could go somewhere special with the large calibre guns.
HLBS has a glaring hole in their line up in that department as all they offer (as far as I can see) are boring Dahlgren guns which, whilst fine for ACW, aren't really proper steampunk to my mind. (not enough machinery!)
My suggestion would be to have a range of guns ranging from the Armstrong guns on HMS Warrior.

to the naval artillery of the turn of the century.

These sorts of guns would look awesome on anything, especially on your aeronefs etc, are not easily scratchbuilt and aren't available anywhere on the market (as far as I know), the closest you can get is world war II artillery, which is pretty obvious to anyone who knows artillery pieces.
So yeah, that's my 2 cents on the matter. Make your shop stand out by fielding these awesome looking guns.
