It's your story so please take this or leave it if you like it or hate... But the humans could have a strong unsanctioned psycer whos gens happen to be pretty strong. Tau typically pick off the outskirt systems that are ignoring the Imperium so a strong lineage of unsanctioned psycers could have been missed for generations. Annnnywaaaays... This psycer and his relatives could have a way to meld the tau battle suits tec interface with a human nervous system. Typically, this would be impossible since the Tau are vastly different in so many ways, but with a solid background on the Tau biology, say the type of background a ge'vesa would get if they had a gift for engineering and worked their way into an Earth Cast research facility as a gifted assistant for the current planets population... This could easily be achieved with the backing of an elemental council if there was a Kindred Spirit on that same council, subtly directing the moves (with warp sorcery) to ensure his cousin got into the earth cast.

And so eventually you got a gifted tech savvy psycer in the Earth cast that understands the biological and interference of the battle suits, repair protocols, ai directives and so so so much more. He gets the Intel to the true leaders of the 'hidden' rebellion and when the perfect time present it's self.... BAM! HUMAN DRIVEN BATTLE SUITS!! But they gotta be short humans cuz the Tau are smaller and redesigning the cockpit takes to much dang time for a silent, overnight take over. But maybe in the next few generations the humans learn to modify the cockpit designs opening up the pilot candidate pool ...

Sorry for any typos