Tiksi How do pokemon forms interact with your rule?
My initial thought is this.
Whether a forme has a Dex entry or not is no longer a useful method to see if the game recognizes the form as more substantive than e.g. gender difference, because almost all formes have Dex entries now. Even the Gastrodons. So my test, unless something else is involved, is whether they subjectively seem to be given a degree of distancing beyond "looks different and has slight difference in personality/etc.".
Rotom formes count (No machine possession v. Machine possession. Supported by BST jump from Rotom to forme Rotom. Formes are Rotom, Heat Rotom, Mow Rotom, Frost Rotom, Fan Rotom, Wash Rotom)
Silvally does count (No activation of type-changing power v. Activation of type-changing power. Supported by Dex entries being for "Silvally" and "Silvally-(Type)". Silvally, Silvally-Bug, Silvally-Electric...)
In-battle formes do
not count (Legality for this only checked at teambuilder. This includes Mimikyu, Cherrim, Morpeko, Wishiwashi, Keldeo?...)
Regional formes do
not count (They don't "actually" have a different name. Regional formes are named as "another one" of the original, just given a convenience designator, be it (Galarian Forme); -Galar; whatnot.)
Toxtricity, Basculin, Polteageist, Gastrodon, Alcremie, and Gourgeist don't count.
Silvally notably gets exactly zero new moves or abilities regardless of form. So it doesn't really matter. Rotom matters though.