The difference between
Meguiar's Final Inspection and Quick Detail Spray, is that the FI does not contain waxes or silicone, where the the QD spray contains silicone. You may not want to use clay lube, as that is more "soapy" and may contain ingredients that you don't want in between your SG layers.
Personally, I used the QD spray between layers when I first started, and had no obvious problems, but to do it right you'd want to use water or FI to remove SG residue.
I hope that this helps. I'm glad you enjoyed the instructions, and more importantly, the results!