they should be fine on the slingshot board. the system binding in itself has quite a bit of flex to it so it should flex with the board. you will need slingshot hardware to mount the bindings which should come with the board. you may or may not need washers to tighten things. washers or trimming bolt length is not uncommon when mixing ANY brand of boots and boards.
as for the fast track type mounting, considering that hyperlites "flex board" the union has the very similar strata track I think it will be fine. http://hyperlite.com/boards-union.php