That should read: Make sure the redtail cat is alot smaller than the aros. If the RTC is 2/3rds the size of the aros it will eat the aros.
What size tank are you planning to keep this trio in. The aros alone will need a 300gal. A mature RTC will need minimum 650 gals.
BTW, the RTCs growth rate is about double that of a silver aro. So, eventually, a mature RTC will be able to eat a mature silver aro.
Again... Should read "Doesn't mean it will 'today'."
When he was 16", my little RTC ate 4 1-1/2 lb channel cats in a week. Enjoy the group while you can. When the RTC outpaces the aros in growth, it WILL, not may, it WILL eat its tankmates.
My little RTC is now over 2 ft and eats whole 2-3lb crappie and tilapia without effort.
No ones mentioned what size tanks these combos are being maintained in. I'd like to know. My RTC has become our hatchery mascot and he has his own 450 gal. Hopefully, Quikset will have completed the 4 2000gal raceways we ordered by the time our RTC outgrows his 450.