It's a nice idea. I think, medium slot should be used, because ram should be huge and immobile, and small gun slots are too small. Maybe gunner will have to mount it, like he does with guns.
What sort of role would the gunner play in mounting a ram? (Not a male sheep )
Unfortunately there is a decreasing number of captains willing to have a gunner on board (which is understandable).
If the gunner was to make an impact, what ammunition types would have what effect?
Well, since the game is steampunk we could do something like pneumatics. So the gunner(whomever is on the ram) would aim it and match the timing for a hit.
As with ammo one could change the ram's head, greased would make it launch faster, heavy would make it hit harder, ect.
Depending on the weapon the way the weapon "fires" could be changed. so a ram would be a one shot deal but a blade would be extended off the hull as long as there is gas(ammo) left.