What would the Old Ones think of the Tau's Philosophy of the 'Greater Good'? Is that okay according to the 'Will of the Old Ones'? Is the Greater Good based on the Will of the Old Ones, or is the Will of the Old Ones based on the Greater Good? Can I make Seraphon models with Tau weapons and armor, or is that too extreme?
BTW I made a post on the Tau forums, advocating for the Will of the Old Ones: https://www.advancedtautactica.com/viewtopic.php?f=1&t=30217
Kind of the old ones are determinist. Everything they made has a specific job and purpose the lizard men do with their supposed to Tau do what they're supposed to everything is in its place. The great plan is less a guideline to the Future and more what everyone's job is supposed to be. So I think that they'd be perfectly fine with the tau doing that as long as it is what they planned for the tau to be doing but probably not everyone
Are we talking about the Old Ones, the Lizardmen or the Seraphon? Because they are not the same thing. The Old Ones are (almost) above the material universe. they have a LONG scheme going on, and I'm pretty sure they can include the Greater Good in it, since it's a ideal that follow the order. The Lizardmen couldn't care less. A thing can be a part of the achievement of the Great Plan, so it will be tolerated / accepted, as long as it doesn't interfere and stays at a safe distance from Lustria The Seraphon are already into the Order allegiance, so they would be more "open-minded" toward a philosopy that encourages cooperation and peaceful life, but answers with a heavy use of violence towards those that threaten it. ...and regarding your last question, there's nothing wrong in some kitbash.
In fairness, we're not so much "in" the grand alliance as much as we occasionally show up out of nowhere and start randomly killing whatever is deemed "bad" possibly saving other forces of order. The only main difference is that we've started to do a bit more active diplomacy compared to the lizardmen. In the old world skink diplomats were exceptionally rare, though not impossible. In AoS we have (semi?)permanent enclaves in some of the bigger cities which mostly keep to themselves. But so far we haven't seen much actual cooperation beyond a Skink diplomat potentially telling the rulers of a city they should do a thing to fight chaos. We haven't exactly seen much in the form of joint force operations or a band of heroes that included us.