Well the 2.5' tank width will not be good long term for the longnose gar. They are likely to reach about 3' in length (possibly bigger), and will need something closer to 4' wide to be comfortable in. You may consider building/buying a pond that's more like 8x4, 8x6, 10x4, or 10x6. Those will all be better sizes for the longnose gar.
The rtc will also need the large footprint as it grows, as it will also likely reach a size of 4' long eventually.
The gar and rtc can coexist, you'll just have to be ready and prepared to separate if any aggression starts to occur.