Re: Limits.
The only limit is your patience in waiting for the thing to churn out such a gigantic image (Unless you want something like 512x512 for 1500 provinces, which doesn't sound brilliant to me).
You might also run out of RAM if you try and generate something like a 32768x32768 map
(Though that won't really stop you, unless you also run out of swap space
)
Summary: Limited only by processing power.
Re: Shotgunning.
Well, currently the program generates two "maps", one is for altitude, and one is for precipitation. The altitude map determines whether a pixel is deep ocean, ocean, land or mountain, and the precipitation map determines whether a pixel is waste,plains,farmlands,forest or swamp.
This means that you can control each set of data separately. If there was demand, it would be trivial to even make a single "map" for each terrain type, though I think that might end up looking terrible. (And imagine the possible outcomes, a mountain/forest/waste/swamp/farm province :S)
I suppose it might make sense to decouple mountains from altitude and make a "cragginess" map.
As for the shotgun effect, you can control how rough each map will be, and you can control what boundaries of precipitation (for example) will create each terrain type, but you will most likely end up with one terrain type surrounded by the other (wastes will always be surrounded by normal provinces [as in, not forest/farm/swamp], because to get from the precipitation required for a waste to that of a forest it must pass through "normal" first).
So in summation, yes you can create a shotgun effect, to certain limits, which can be changed by popular demand.
Re: Logical provinces on a found image.
Unless you generate the map with the generator, it won't be able to determine the terrain type of each pixel, so it won't be able to draw borders based on logical boundaries (such as the edges of mountains), but it will be able to draw borders between provinces based on distance (it might look something like
this), and it also won't be able to write down province type to a .map file (though will be able to write down neighbours).
However, if when you made your map you started with a layer that describes the type of each pixel using one color (for example, if you started with something that looks like what my program outputs right now, but painted over it with effects/sprites/fluff/etc.), then it would be trivial for me to write a program that would take that base image and convert it into my own format, which would then enable you to pipe that into the province placer along with your finished image, and it will be able to work its magic.