a senior developer, by definition, will hit the ground running with mostly anything you use. that's the senior part in senior developer.
also if you believe that people need years of use to be good in any language/tools/framework you need to figure out how to attract better people.
also, the question you need to ask yourself is: do you want to build something with a technology you've selected and think it's the best or do you want to have someone that can pick the right tool for the job pick the tech? sometimes, not building something or various parts of something is more valuable that building something that you don't need fast.
also, the question you need to ask yourself is: do you want to build something with a technology you've selected and think it's the best or do you want to have someone that can pick the right tool for the job pick the tech? sometimes, not building something or various parts of something is more valuable that building something that you don't need fast.