I think you should be able to move the contents of your old tank to the reefer without any loss of livestock.
Hopefully someone with more experience can give you a second opinion.
Short answers:
How long would they be ok in there for though?
A few days should be fine. You might get away with longer by doing water changes or adding additional filtration. My concern with a longer duration would be providing enough food and dealing with the waste.
Event though I am using everything from my current tank and some extra sand and rock wouldn't the tank still have to cycle before I put the livestock back in the new tank?
I didn't detect any spikes when I did this, but every tank is different so it's possible you will have a small cycle (my tank was also 9 months old). Personally I would avoid putting back unwashed sand or adding new rock for a couple of weeks. You could certainly hedge your bets by using a bacterial additive such as
Seachem Stability or
Dr Tims One and Only to prevent any spikes.
Longer answer:
Before moving the tank, I created a simply plan consisting of the steps to take, what equipment I needed and realistically how long each step would take (particularly making RODI water / mixing salt). In my case, I was trying to buff out a scratch in the glass which added (wasted) a lot of time, so you could probably do your move over a weekend.
When I moved my tank I put the fish, rock and filter media in one tub and the corals in another. They were in the tubs for 3 days with a heater and wave maker (no sand, no skimmer, no lights) and one small feeding of flake per day.
Once the tank was moved, on day 2 I partially filled the DT with enough new water to cover the rocks, added the rock along with a heater and wave maker, then added clean sand and left it overnight to settle. I spent the better part of day 3 adding the livestock back to the tank (acclimating everything as I went) and then slowly topped off the system with the old water from the tubs. I tested for ammonia/nitrite/nitrate every other day for the next week but did not see any spikes.
