Can I mix galvanized and black (iron) pipe when plumbing for new gas service?

I’m getting new gas service (Nat. Gas, M425, 1-1/4") installed. Big Box Store doesn’t sell 1-1/4" black pipe (only 1"), but they do sell 1-1/4" galvanized. Can I run galvanized for mainline (from exterior protruding nipple for meter connection, then run up and through the attic) and branch off using smaller 1" black iron pipe, manifolds, and fittings or CSST and fittings? I’ve heard common folklore regarding galvanized, but I’m looking for cited code violations.

I’ve tried to thoroughly review UPC and IRC codes, but everything seems to point to no issues using black iron, galvanized, or CSST [1], but I couldn’t find anything mentioning a mix of them.

What I know so far from my research:

1. Galvanized is no longer a risk when used with NG because modern zinc coating processes no longer cause flaking and NG composition is no longer corrosive. It can be used outside or where moisture is present.
2. Black (iron) pipe can’t be used outside or where moisture is present.
3. CSST must be properly bonded and installed by a certified person.
4. There is no galvanic corrosion/reaction between galvanized and black pipe because they’re both made of "soft steel", galvanized is simply coated with zinc.

Code should be relevant to my locale: Snohomish County, Washington State, USA.

[1] IRC G2414.42(403.4.2), ASTM A 43/A 53M: https://up.codes/viewer/washington/irc-2015/chapter/24/fuel-gas#G2414.4.2

This specification covers seamless and welded black and hot-dipped galvanized steel pipe in NPS 1/8 to NPS 26