i wouldn't be so quick to say that Miguel ; depending on the poly, both species of fish could have very different growth rates that might end up jepordising the longevity of the poly. in my experience hiding spots does not work in the long run once a gator gar decides he wants to eat something (yr old rck would know this

), especially if that something needs to constantly come up for air.
i also wouldn't say that a gator gars hunting is limited to top dwelling prey, for the most part all my gars are opportunistic feeders and will attack and feed from all areas of the tank.
i personally would (1) grow out both fish in seperate tanks till the gators' growth pleatus, between 14-18" and wait till the poly is of a similiar size before mixing them, or (2) just wait till the gators growth stablizes before getting a large poly.