I was looking through a tau codex and noticed that that path finder teams are required to take a devilfish apc I started thinking about how to use the transport to your ad vantage other than just ferying people around and I came up with this strategy for your scout move you turbo up 18 inches to get the best position but if you disembark the pathfinders you canot use their markerlights the first turn as they are heavy so, as long as the transport is not destroyed in the first turn if you had ended your turn in cover and the devilfish is equiped with sensor spines could you lower the transport into the cover obscuring it and bloking line of sight to the pathfinders.

OK, I see at least one problem here, maybe a couple. First, you have to take a "normal" move during your scout move. Second, I can't remember (and don't feel like tyring to find my rulebook) if a unit with Scout gets into a vehicle during their scout move, does the vehicle get scout? And thirdly, it says that when you take a devilfish, the pathfinders have to be outside it, so even if the vehicle did get to scout when the pathfinders jump inside during the scout move, they wouldn't be able to disembark again until your 1st turn. I just recently learned that you can't embark and disembark during the same movement.

I have just read the tau pathfinder entry in the codex and it says that the pathfinder squad must take a devilfish apc but nothing about their method of deployment

The pathfinders can choose to begin their turn inside the vehicle. If they do choose to then the vehicle gets the scout move and it can move and they can desembark in the scout phase. People have been doing that sort of thing with the scout move since the beginning of 5th.

Now as for the cover save thing the DF doesn't have to lower to give atleast a cover save. Not sure if you really want to land the DF though to obscure and block all line of site so I would recommend not landing it but leaving it infront of them until it can be moved out to go carry others across the battlefield.