Or put simply, the exams and the Skills test must be in date when you apply. The RT written is mandatory, but the practical is not however; if you don't do them all together, the RT exams revert to their 12 month validity LASORS B1.2
Another point you should consider is that you medical must also be valid, have an accident and lose the medical then everything could become null and void so don't delay sending in a completed application.