Thank You for the helpful detailed explanation, I would go with the dial gauge route.

Do you think this dial gauge is good enough for the job?:

Mitutoyo 513-405-10E DIAL TI, BAS, STD 0.2 mm, 1 μm Accuracy, 0.002 mm (2 μm resolution), Yellow

https://www.amazon.com/Mitutoyo-513-405-10E-Accuracy-0-002-Yellow/dp/B079HGWCHH/ref=pd_lpo_1?pd_rd_i=B079HGWCHH&psc=1

or this one:

Mitutoyo 513-401E Dial Test Indicator, Basic Set, Horizontal Type, 8mm Stem Dia., Yellow Dial, 0-70-0 Reading, 40mm Dial Dia., 0-0.14mm Range, 0.001mm (1 μm) Graduation, +/-0.003mm (3 μm) Accuracy

https://www.amazon.com/gp/product/B003UATP1W/ref=crt_ewc_title_dp_2?ie=UTF8&psc=1&smid=A2ZFPLI33HB8U5

I think the first dial test indicator is better since it's accuray is 1μm but has 2 μm Graduations.

Should I get a granite square or granite straight edge or both, also will it aid me to properly install the linear rails? If so, how exactly do I properly place and align them to the surface plate to help me test the straightness and parallelism of the rails?

I have many questions. Would like to gather information before purchasing things to start this project.

Thank You.