Tank Mid-Cycle, a few questions

Hi, I've been away from the hobby for about ten years and am in the process of starting a new tank.

It is a 40 gallon Bare Bottom with a 20 gallon sump. I've got about 20lbs of base rock in the sump with more on the way.

Three weeks ago I got the plumbing sorted, added some Instant Ocean, then a piece of shrimp from the deli counter. Most of the shrimp is still there, slowly rotting in the sump.

Here are my current parameters:
S.G. 1.024
Temp. 77.5
PH 8.4 +?
Ammonia .25-.50ppm
Nitrite 5ppm
Nitrate 40ppm

The testing is a bit tricky for me as I am slightly color blind, so I may be a bit off on my readings.

I've got a tiny bag of carbon in the sump to clean out any plumbing glue residue. Is this good or bad for the cycle?

Should I get my skimmer going?

When should I do my first water change?

Is it time for a piece of live rock yet?

What can I do to control the PH?

Thanks everyone. I'm starting from scratch here and re-learning as I go.

My end goal here is to have a frag tank starting with zoas. I plan to get a black molly to control the hair algae that I hear is going to grow on my eggcrate and a small clean up crew, but that is further down the line.

Thanks again! Take care.