I don't know Texas mt laurel but in general seedling stage is the safest time to reduce the main root. Even species that are traditionally difficult to root prune as adults can have seedling roots chopped aggressively and respond well. I generally root prune seedlings well before they reach 12 inches tall to prevent strong tap roots. As soon as they are big enough to handle is my preferred timing for first transplant and root prune but many will still be Ok for later transplant and root reduction.
Thanks For the input.