All training is velo training. You can't build velo unless you build strong body stabilization. Your body won't unlock itself and allow you to throw hard if it has to protect itself when in the positions you need to get to transfer energy effectively. It's very important to have an evaluation done of your movement patterns and core body strength to know what to work on, to make sure things like scaps and t-spine move correctly because they are properly supported by the right muscles, etc. to throw as an overhead athlete. Building raw strength without these things is a recipe for problems and frustration.
So to answer you questions, I would say you start (in I think your terms) velo specific training, when you establish that you've addressed any and all movement pattern issues due to weaknesses in specific important muscle groups.