You're getting good advice above overall. I'd recommend to try a gator gar with an RTC as you had been planning, because I love monsters (hence my name) and like your enthusiasm, but I've only kept them together for a few years and had no problems. RTCs were 2.5ft. Gars were 2.5ft. IDK what happens when the RTCs are 3, 3.5 and 4ft while the gars remain the same or lag behind at 2.5, 3. 3.5ft, but if you are buying a color morph RTC then again, you may be right and they would grow to smaller maxes, no one really knows because the plat RTCs, phantoms, etc. haven't been around long enough yet. I'd expect them to grow slower and smaller and live shorter as their genetics is compromised. I am not a morph fan at all though.
wednesday13
is
Gars seem to not eat gars, even of different species, as Hao said, IME too.
TSN and TSNxRTC of 2.5ft of ours ate some 2ft FL/spotted gars and longnose gars of ours before we knew better... but again, not RTC, which I cannot promise would never happen.
Yes personalities of RTCs and all fish vary but in terms of preying, it only affects chances and times, it'd not be safe and not safe categories.