Painting new boards installed on old front porch
In my humble opinion, I'd wait for the new wood to dry out and then pressure clean everything. There will be some seepage/residue from the pressure treated wood that you'll want to remove before painting, plus, it will get dirty while drying. Same with the floor, if you pressure clean it now, it will get dirty while the new wood is drying out. After pressure cleaning, wait until both woods are completely dry before painting.

Get your new and beautiful paint now and follow any additional instructions that are included with it.