is it possible to do a plastic canvas work that is one count and use another higher or lower count?

Yes. It will change the size of the finished product. If the pattern calls for 7 count canvas, that's seven holes to the inch and you use 10 count canvas, that's ten holes to the inch, your finished project will be that much smaller.

I have a soft ware program that makes patterns for me, and tells me the finished size.

You mean the canvas is a higher or lower count? Yes. It's just that the size of the work will be different. Let's say you're making a plastic baby block: 10-ct. would be bigger than 12-ct. Smaller work (higher count) can also be more fiddly because you are trying to fit the same number of stitches in a smaller area -- so if you have bad eyesight, arthritis or are just a beginner, you might find it easier to work with lower-count (bigger canvas).

Oh, and if it gets too big, you may have unattractive holes in your work unless you get bigger thread/yarn. Vice versa, if you get very small, you may want to switch to smaller thread, even embroidery thread.