red tails tend to top out around 10" in tanks, spotted gar will more than double that, gar tend to try to eat things that are much less than a third their size. if they grow up at the same rate than no problem but gar grow fast in the first year. If you end up with an 18" gar and a 6" cuda than you have a very short term problem. Just keep an eye on the relative growth rates. if the cuda stays at least 1/2 the size of the gar you have no problem.