"any special rules related to Roth 401k?"
An entire section of the tax code, section 402A, covers designated Roth accounts in qualified retirement plans. From that the IRS developed sections 1.402A-1 and 1.402A-2 of the Code of Federal Regulations.
https://www.law.cornell.edu/uscode/text/26/402A
https://www.law.cornell.edu/cfr/text/26/1.402A-1
https://www.law.cornell.edu/cfr/text/26/1.402A-2
The IRS has provided additional guidance on In-plan Roth rollovers in Notice 2010-84 and Notice 2013-74:
https://www.irs.gov/pub/irs-drop/n-10-84.pdf
https://www.irs.gov/pub/irs-drop/n-13-74.pdf
"Anyone aware of the IRS writing how the roth 401k is the same in treatment during rollovers?"
The taxable amount of an In-plan Roth Rollover is explained in Q&A-7 of Notice 2010-84.
