>>96224595
They have slipstream drives that can punch through and make new wormholes, linking wherever the fuck they end up to the Empire. So far T'au have been found near Baal, on Vigilus, and more. It was a business decision by GW to allow peeps to use t'au no matter the where in the galaxy the warzone is.
>>96224592
It's a lot when you're only one person. You're usually not gonna need 100 worlds for a game, so keeping a sector's worlds to ~20 lets you focus and put a lot of detail into things.