the best would be to find a temporary home for the fish or if you have to add him to the aquarium 24 hours after mixing the salt and do very frequent water changes over the next month at least - i'm guessing 1 every week, preferably several small changes several times a week. Add the rock and the fish together, it will act as a Berlin System and help while you get things sorted out.
Important, change water between the bucket and the aquarium slowly over an hour or 2. I suspect the water in the bucket will have amonia, because of the limited filtration. Amonia changes to amonium when there isn't much aeration. Amonium is harmless, but with a sudden ph change which will occure with fresh sea water it quickly changes to amonia. Your fish will not eat, and with in a week be bent over and die. This is what often happens with fish in a bag from a pet shop if not introduced properly. No way around it but to slowly change the water, dilute the amonia in the water (and hence in the fish).
This is not ideal but it is what I would try. No new fish for 3 months. Not negotiable.
It your fish survives, it will be because of live rock, your skimmer and your small frequent water changes.
Can you fix your skimmer up to your bucket. That could help. Its a decent skimmer and it will take alot of waste out of the water. Will help for the mean time.
GOOD LUCK