[Marlin65]

As far as your cycle did you add anything to start it? like a dead shrimp.
If you used uncured rock it would have enough dead stuff on it to start it but if you used mostly cured there will not be much to start it.

Add Kalkwater to bump your PH and calcium at the same time just make sure to add it slowly and preferably at night when your lights are off.
Or you can use buffer to bring it up.

Your saltwater will keep a long time if you keep it dark and cool. A few weeks with a heater and air in okay though. You should leave it for at least a day to mix it's better that way than fresh made.

Rubble rock is just small pieces of live rock you could try the store or just use bigger pieces. Does not make that much of a difference. Smash it with a hammer to make it fit in the canister.

On a 27 gallon just do the water changes if you want to buy the skimmer later. The skimmer just lets you do less water changes less often as it removes the crap. Don't do any water changes until your cycle has finished though as that prolongs the cycle time.

I started with adding a few soft corals first before CUC and fish.
as they are a bit more forgiving. Most people add CUC then fish then corals.
But I would say try a mushroom or two to test they are cheap and are a good indicator of your water quality.
Usual time is about a month but you started with rock from an established tank and it is possible you have already gone through you cycle. I cycled mine in two weeks but same as you had established rock and only had a small NO2 spike no Ammonia. I then went to no NO2 with 10 ppm NO3
My tank is now a month old and my NO3 is at 2.5ppm
Have 2 clowns SPS and some soft corals all doing great.

Hope that helps don't forget to test magnesium as thats tied to your Calcium.
Most salt does not have enough of both so you need to add it to get your levels where they need to be.