by VATF do you mean vittatus african tigerfish? if so, i would not keep them with gars...they are too skittish and no gar can resist a silvery, fast-moving fish. they will get shredded and abused no matter how big they are, and if they are big, they may set off the gars and you could end up with gars with broken backs. if this is what VATFs are, i would highly suggest against the combo.
and, as stated above, no gar has a wild max of 18", they are all about 33" and up for their wild max sizes. however, in aquaria, several gar species (such as spotted/Florida) will top out around 18-20", generally speaking.--
--solomon