i've kept gars with grass pickerel before...only works temporarily and with less aggressive gar species (FLG should work, Gator/Cuban less so). the growth rate of the gar will eventually surpass that of most GP's in captivity, and pickerel have no armor defense and are prone to bacterial infections, so if the gar puts a nasty bite on them (as can often be the case), the GP is very difficult to treat. this is based on keeping the combo (or direct connection - roommates, colleagues, etc) several times.
if you have the choice, i would go with the gar and skip the pickerel (or vice versa) - together is temporary, and i know that's what you are considering, but their growth rates being unequal makes it for a tenuous combination even temporarily--
--solomon