You can take the ATP written after you complete the ATP Certificate Training Program. But you have to have 1500 hours before you take the ATP checkride. The written exam is valid for 60 months. So if you are sure you will have 1500 in 5 years go ahead and do the ATP CTP and the written. There is no "frozen ATP" in the FAA system. You will still be a commercial pilot who has passed the ATP written.
You can add a type rating to a private, commercial or ATP certificate. The FAA requires all type ratings to be flown to the ATP standards. So if you add a type rating to a commercial and then later get an ATP, the type rating will move up to the ATP level without another checkride.