Although I understand the reasoning, that only one grid can exist in a group, and therefore the game does not allow you to add other grid items into a group that already has grid items, I find that in practice the game does not even follow its own rule.
I have many large groups (some just hundreds others thousands of items) where there are in fact multiple grids in effect. I will simply be building along thinking everything is in line, then go back and build another wall or floor near one of my earlier placed pieces, and see that they are somehow slightly out of line in the X or Y direction compared to the earlier placed pieces (I also find it with the Z vertical direction, but items can be moved on the Z axis more freely so it is less of an issue). Fortunately I can use the "select grid" button and click the item I want to use as reference, and then move the other item, and it will move onto the first item's grid instead of being on a second grid within the single scenery item. From this, it is clear to me that two (or more) grids can exist in the same "single grid" capable scenery group, and the game handles it just fine, so I would really appreciate if instead of the game only accidentally giving me multiple slightly offset grids (which is a huge pain when you thought everything was being made in line, and it turns out it is no longer on the original grid despite being in the same group) it would be very nice if the game would let me also intentionally combine completely differently gridded items.
The "select grid" button does all the work needed to deal with multiple grids in one scenery object, so this would not add any problems on the user end (well, not any more than it already is to learn the system) and the game is already storing multiple grids to a single group, so aside from not having programmed the behaviour to intentionally allow multiple different grids in a single group the game is clearly capable of providing this function.