(completely ignoring fluff here- I simply don't know enough about Tau fluff)
At 1250pts I would probably not go for guard allies as you'll need to spend a couple hundred on troops and HQ before you add in fun things... but if pts are no issue I would take this:
Primaris Psyker- psychic support you wouldn't otherwise have
Veterans- loaded with plasma or melta depending on what you need
Vendetta- easily the best anti-tank/anti-flyer flyer and can trasport your vets (and hq) to where you need.
Marbo- because he's immense
That's about 400pts and leaves you a heavy support option (I love griffons just to blast the shit out of whatever you need) but if you want massive impact from your allies then adding in a manticore isn't a bad plan: giving the vets demolitions would then mean that for ~550pts you would be potentially getting 5 S9-10 large blasts for 1 turn plus 3 TLLC, 3 plasmaguns/meltas and your psychic powers.
Alternatively, if worried by TEQs you could take a Leman Russ Exterminator for its plasmacannons... damn the IG dex has some shiny toys