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.