Basically keep mine and add your #6 into my list as like #3 (making my current #3 to #4)1) Do they visually appear different?
Yes - Move onto next question.
No - Do not tier them separately.
1 - Pointless criterion. If they're a forme then they have a visual difference.
2) Do you have the ability to change forme in battle?
Yes - If a forme requires another forme of Pokémon in order to be brought into battle, and that other forme is not legal in the tier, then do not tier them seperately. Otherwise, move onto next question.
No - Move onto next question.
This is superior, since the tiering separation is independent of how the Pokémon end up. I think it makes it clearer w.r.t. megas, as well as stuff like Darm-Z, Castform formes, etc. as you mention.1) Do you have the ability to change forme in battle?
Yes - Any forme which requires another forme of Pokémon in order to be brought into battle (e.g. think megas) then that forme can only be brought in a tier where the base forme is legal. Base forme here defines the forme which is brought into battle initially.
No - move onto next question.
3?) Does the forme force a specific move on sets?
Yes - Do not tier them separately.
No - Move onto next question.
4) Does the forme originate from an ability?
Yes - Do not tier them separately.
No - Move onto next question.
3 & 4 - not nessecary. If something would be Do not tier separately, you can just not ask the question, and for those Pokémon, they will reach the final question, and it will say to not tier them separately. The simpler this is, the better - 3 steps is easier to understand than 8 steps.
5) Is there a change in typing between the formes?
Yes - Then tier separately, and treat as separate Pokémon.
No - Move onto next question.
(note to other readers, this is the same as my #2 question)
6?) Do they have different abilities?
Yes - Then tier separately, and treat as separate Pokémon.
No - Do not tier them separately
I like this addition.
7) Do they have different (base?) stats?
Yes - Then tier separately, and treat as separate Pokémon.
No - Do not tier them separately (unless differing movepools is accepted as criterion)
(note to readers - this is the same as my 3rd criterion)
8?) Do they have different movepools?
Yes - Then tier separately, and treat as separate Pokémon.
No - Do not tier them separately.
8 - No.
Edit @ Lutra:
No we keep species clause! The complexity in viability is a part of the result. I think changing Species Clause to Forme Clause would destabilise ORAS 1U.. also in GSC you would be allowed 2 Snorlaxen (with a Shiny and non-Shiny one) if that went through.