I'd be tempted to make those slots as wide as you can get away with to help you get the strings on and off. I'd have been tempted to use locking tuners (locking from the bottom types) for the slots just to make that easier. You could use locking tuners throughout but they add a lot of weight to the headstock, not something you want when you are already starting out with 12 tuners, so 6 locking + 6 normal would be a decent compromise (unless you can find really light alloy ones).