1. I think it is possible to maintain at last only; if there was any package number skipped initially (like 4, i.e. 1,2,3,5,6,7) then it was possible to insert there in between.
2. Your cycle short text will be unique & independent like 2M for 2 monthly.but overall package will be restructured and already assigned package in plan will not Exists, so it will be required to reschedule the plans. you should follow single cycle plan as it is required for some equipment only.
3. there is no bad impact to use both, it is general practice to use according to requirement all over the business.
enjoy...