To answer the first question on if it is faster to build the tank or trust RNG, building is always the way to go. RNG spreads its blessings few and far and even though tanks can drop from zones you should never count on it happening (and if it does, then lucky you!). Now with the second question on dogtags and Fate. I myself have not fated a Blue or Purple tank so I don't quite know dogtag costs for promotes but a friend told me his KV-85 (Purple) promote dogtag cost was 90, so I will roll with that number for now. Now Fate is a cruel mistress to Blue tanks because costs are generally higher then Purple fates and Gold fates, so i'm taking shots in the dark here, but dogtags costs for fate promote one on a Blue tank could be 90 tags then the second promote if they increase costs like they normally do in promotes, that could be around 120 tags. So this guess brings in around 200+ tags for you to get before 18 (if these costs are right, and they are probably not). Can you reach that much before 18? Sure you can, that's a very long term goal but you definitely can. If you asked me, I would say using a pin on a Blue is not worth but hey, Waifu > All logic