It'll vary from park to park, company to company, but for a large new ride, they'd almost definitely want to get more experienced staff involved. They can have new staff too, of course, but they'd want staff who have a proven track record to get involved.
If staff get to ride Fly, it'll be the perfect time to get staff to experience what it's like with people who are new to the area. It's as close as you can get to practicing with the public without having the public there.
As for the staff adverts and new staff, they'll likely be there to replace staff who go to Rookburgh, as well as extra staff (which they probably need for Winteraum anyways). New staff can be put into the action very quickly once hired: have an induction into the business which gives you basic knowledge, rules and H&S details and a department induction which gives you specific job knowledge and training. From there, you're out there, possibly shadowing staff depending on the role, in the thick of it.
For working on rides, even at companies with the most rigorous and strict training procedures, you can go from 'very first day' to 'batching guests, checking bars and pushing buttons' in like 3 days, if needed.
