@Latecomer
I have nothing to say on the toy aspect.
For personalities, you could always have them a combo of different traits like maybe there could be a Warrior Princess who obsessed with fashion and elegant brutality.
As well as you could always have certain characters be side characters instead and give a bigger focus to said ponies.
The benefits to having less characters at the beginning is that it could allow you delve deeper into certain characters. Also it could create less tropey characters and more multifaceted characters.
As for which of the Mane Six to eliminate, I don’t know but make sure you know what roles each one character play in the group dynamic. And you can always give something like the clownish personality to one of the existing four.
My method has its own disadvantages but I think it might be worth considering.