@AGypsyRed If you're want to sell patterns, what I would suggest, make the sizes standard, like they do with commercial patterns. And then if someone had a their own sloper, they could easily fit your pattern without having to worry about alternations.
It's just easier to change a pattern if it's closer to your size, but you can work off them being an off size, just means some extra work would have to be applied.
Instead of investing into a program, I would say...just worry about making clothing designs. You can tell people, "hey, you can size these using a sloper" or whatever. You can look at pictures and be like, "oh, I can design that," and draw it out or people to use.