very nice tank mate

i always get my fish as babyes and keep them in a 30-40g for a few months then move them onto their bigger tanks as a 2-3" rtc just doesnt look right in a 120g get what i mean
tbh RTxTSN hybrids grow much bigger than RTC as they where specifically bred to grow bigger for the food market thats why they where originally made
i no it sounded a lot 1" in 3 days but really its not impossible as each individual specimen grows at a different speed so it can average also it depends what hes feeding it like if their getting feeders often they will grow quicker
like i dont feed feeders all frozen or prepared and my RTxTSN growed to 18" is less than 6 months and thats just with frozen so if your feeding feeders they will grow even faster
but a bit of advice on the tank i wouldnt trust a undergravel as they just wont cut it on their own and i no their all babys but they still have very large bioloads and with your stocking i can imagine it will be a strain of the bioload so i would add 2 or even more mini internal power filters or try and rig up a small canister