a lot of this is common sense. if the gar is not big enough to feasibly take down a whole silversides, then don't feed it whole silversides. at the same time, the feeder fish size is different depending on the supplier, package, or just the size of silversides you buy. if you are feeding pretty small silversides, then your gar can likely take them down whole if it is 6" or larger (silversides on the "small" end of the spectrum are around 2-3" long). for larger silversides, either cut them into pieces or just wait until your gar is larger...at 9-11", most gars can take down the silversides readily available at LFS...use common sense in judgment though, and if you get a silversides that appears to be too big for your gar...chop it up.
all that being said, there is no need to cut the head and spine out as these contain nutritious materials for the gar as well--
--solomon