a single FL gar (which is what you'd be getting if you purchase the typical "gar with spots" from an LFS...spotted gars are generally not available in the hobby) will not require a longer tank than 4'...it's not going to grow to that length in captivity (or really otherwise). the loaches would be the main fish at risk...they grow more slowly than the gar, so it may quickly surpass the loaches...although they MIGHT be too big to swallow, either one or both fish may die if the gar tries to do so.--
--solomon