My plan for adding a sub panel:

Residential house
Main panel is 200 amp Siemens (in place inspected)
Sub panel is 200 amp Siemens (to be added)
Connected via pipe nipple metallic through a 2x6 stud
Green = #6
Red / Black / White = #2 copper
Circuit breaker from main to sub panel is 100 amp
Sub panel is going to be used ONLY for some standard basement outlets no 220v

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Question 1: Anyone see anything wrong with my shopping list?
Question 2: Is it okay to have a 200 amp main and an identical 200 amp sub panel?
Question 3: Does the metal pipe nipple connecting the two boxes need to be grounded via a grounding lug on both ends that goes to the two ground strips? I forget what you call it but Home Depot said no but an electrican said maybe?

Will this pass?

1. Have to check wire sizes. There needs to be the ability to isolate the neutral bar in the sub panel. I'd clearly label the panels. Main and (Subpanel). It's OK to have a 200 Amp disconnect even though the sub is fed from a 100 A breaker. You might consider a conversion kit to convert to a "main lug" style panel if available.

2. Yes, see ans 1.

3. You should tie the grounds of both panels together. The neutral to the sub panel must not be connected to ground. The neutral bus and the ground bus are two separate entities in the sub-panel. Mentioned in #1. Clarified here.