I'm creating edits of Lilith's Solitude blend that have top surgery scars, and obviously, I wouldn't want to have scars for non-transgender sims. However, custom skin is the most dominant, isn't it? How can I change that and make the skin non-inheritable? Can I do that with SimPE?

You can't make skins non-inheritable, but if you assign them a genetic value according to this tutorial then they will have the same chance of inheritance as any other skin.

If you assign a skintone the highest or lowest genetic value then it's very unlikely to be inherited unless the other parent also has the same skintone, but any skintone between the parents' skintones could come up - so for example, if you set a scarred black skin to 0.1 and that sim has a baby with an S4 sim, then the baby could come out S1, S2, etc.

If I were doing body scars, I'd do them as overlays (similar to how body hair and tattoos are done), not as skintones, for this reason.

I would have them as body overlays. You could look at one of the boxes, there is dirt, hair, tattoos around as overlays. here is one of them http://www.digitalperversion.net/ga...p?topic=16300.0 A scar box would be very handy if you did a variety of scars. You can do male and female and could add a Caesarian scar, facial scars, lots of possibilities without any worry of inheritance.

Making tattoo boxes isn't particularly easy. If you don't know how to make them, you can download an overlay box you don't intend to use as-is in your game, and switch the textures with your own.

body overlay here
http://modthesims.info/d/239851

tutorial here for how to make your own
http://www.modthesims.info/showthread.php?t=239882

or do what simmer22 said, get ones you don't want and swap the textures