- Can you wash the car immediately after a respray? If so, are there any shampoos that should be avoided?

Yes you can wash it immediately, so long as the shampoo has no wax / gloss additives that could prevent out gassing of the paintwork

- I assume you should not polish/wax/seal the panels for at least 6 weeks - is this true?

It is OK to polish the paintwork, however the product left on the surface must be removed. So that means hand polishing is pointless. Machine polishing to perfect the paintwork is OK so long as its had the wipedown after.
Waxing, sealing, or applying anything that can hinder out-gassing needs to be avoided for 30 days, unless your painter says otherwise and is willing to stand by that should any pop outs occur due to out-gassing... which results in paintwork looking like this...