As always, not everything can be a blanket rule, and in these instances are where tabber creates more problems than it can necessarily solve.
Essentially, I intended tabber for those who have been altered in a fundamental state of their being; Toa being the predominate example, and for crazier ones like Mata Nui. Naturally, these leads to cases of grey areas where we have to define what an altered state of being is and whether or not it merits having tabber based on the information present as a whole. Some argue that, regardless of information, if we know of a change in state (ANY change in state) it merits a new tab. I don't particularly like this, myself, but my reticence leads to certain conflicts, where we have Karzahni (who is an exception unto himself) with a Pit Mutagen altered form tab and not the Barraki or Piraka.
In the very specific case of the Av-Matoran, I don't think it merits having a second tab based on their appearance, because that's a constant; they still look ugly because they have received surface level changes in their appearance. That said, we have Takanuva, who received multiple changes to his state of being (half-shadow, size enlarged); if the Av-Matoran WERE to have a second tab, it would be based on their Shadow-Matoran states of being.
And going back to Takanuva (as well as Gresh, and to a certain extent the other Stars sets), here are brand new forms, but ones that are entirely cosmetic in nature, therefore they don't need new tabs. However, using some of the tabber logic, people would say to switch out the current Takanuva main image with one of his Stars "form", which is technically correct, but something I still oppose, for (some of) the same reasoning as not including the Piraka and Barraki extra tabs; these are the form(s) they people know them best in, and the most holistic representation of the character.