The basic rule is that if the two types to be cross-grafted are similar either in the shape of their leaves or the appearance of their fruit, then it is permitted to crossbreed them. There are some exceptions to this, such as if the taste of the two fruits is very different. (Rambam, Hil. Kilayim 3:4ff)
Applying this rule, then, to your cases:
Rema (Yoreh De'ah 295:6) specifically states that grafting two varieties of one species - his examples are "black with white, or small with large" - is permitted. So presumably this would include the apple cultivars you mentioned.
Nectarines are actually not hybrids, according to Wikipedia. So they wouldn't be an issue.
Nectaplums might be, though, since nectarine and plum trees don't resemble each other in any of these three aspects.
All of this applies only to cross-grafting them. It is permitted to plant different kinds of fruit trees together, and similarly trees with vegetables (Yoreh De'ah 295:3-4).