If the new clown is 1/2 to 3/4 her size, and has always been with a bigger clown, you should be fine. Protandrous species like clownfish are born as sexually immature males, then change to males, then to females.
In a collection of juveniles, the biggest clown will become a sexually mature male, and then a female. The next biggest juvenile will become a male, and the rest will stay juvenile until one of the two top spots becomes vacant.
If you got a pair of juveniles, they'd fight with each other until one outgrew the other and became the male.
Were it me, I'd get a known juvenile (captive raised if not captive bred) that's no more than 3/4 her size.