I've never caponized, but I've bought a bunch of assortments. I've butchered at all different times. If you need the testicles to be decent sized to be able to find, the mixed breed assortments could be tough. Different breeds mature at different times. I've butchered Brahmas that were huge and had bb sized testicles. Wyandottes the same age were the size of plums.
The best thing I found to do with leghorns is butcher them when they are big enough to tell apart. There is not much meat but it is tasty. You aren't going to gain much by growing them bigger. I used to hatch about three hundred leghorns every year, and sell the pullets when they started laying. We ate a lot of leghorn roosters. They are a true production machine, they lay like crazy.