As above, stainless steel isn't affected by any commercial gun cleaning product, use what you want.
Also as above, buy a Lewis Lead Remover kit from Brownell's even if you only shoot jacketed bullets.
The Lewis kit has a special forcing cone cleaning tool that not only removes leading from the cone, it also removes carbon and copper build up. A badly fouled forcing cone can crack as was found out with the S&W "K" frames used with hot 125 grain loads.
While you're ordering from Brownell's buy a couple of bronze chamber brushes. These are much faster and better for removing fouling from the chambers.
DON'T use stainless chamber brushes, these can harm the chambers.
For holsters, any holster made for an original 4 inch Python will work, but depending on the design, the 4 1/4 inch 2020 Python barrel may either stick out the bottom slightly or won't seat fully in a closed toe holster.
Some makers are now making holsters specifically for the new Python.
A good one is El Paso Saddlery.