Hey Pelenor welcome to RA! Pull up a stright jacket and make yourself at home in your padded cell
Much like Titans in the Imperial organisation, I would expect that such Tau superheavies would be in a "need to have" basis like you say. Let's say in our little expanded universe that the Tau have access to a range of heavy/superheavy ground vehicles such as this. Here's how I would imagine it would work:
For much of the time these ground based superheavies go against the grain of standard Tau military doctrine, however there are situations where the extra heavy support is necessary. The Tau military structure is quite dynamic, changing its formation and grouping as required, thus there is only one real position for any kind of heavy support contingent: at the sept (or planet) level.
Not all septs are large enough (or military focused enough) to support something as intensive and powerful as a superheavy support contingent and thus only the larger and more militaristic septs would have them. It would therefore be common for the superheavy contingents to "loan" superheavy cadres as required to military campaigns, whether their normal cadres are active in that campaign or not. This is quite similar to the way that the Adeptus Mechanicus and Imperial Guard work.
This then would allow you to paint your kit in any colours you like, although the colours of a more prominent sept would be more correct.
Like that?
I do love the idea of somebody painting theirs in T'au sept colours though. I know Warhound won't be...
Oh yeah I can make that....
