Gregg:
I've had two bad experiences introducing independent adults into trios. I had a female in the first trio that absolutely would not stop attacking another female introduced into the group.
I made sure that there were plenty of basking and hiding spots and that there were several obstacles to break up the lines of the site. Didn't matter, she was actively hunting her down and attacking her.
The second experience involved a different female and the resident male attacking an introduced male. Same kind of deal - relentless.
Admittedly, I am not a breeder and my experiences could be the exception to the norm.........
As for sexing them - when they are young - I'm lost.... and I've talked to some people who claim they are able to do it and some who are not. This leads to some confusion in the market place. I've seen more missexed ackies - especially now that they are gaining in popularity.
So if someone is new to ackies, I still think it''s in their best interests to either start with a young group or buy adults or subadults from a very well established and trustworthy breeder who can sex them as you described - a lot of people try doing by everting - which often leads to the mix up..