It might be worth while to add SeaRAM to the Phalanx pool. I could certainly see value on some of the bigger ships like the LHD, AOR, and possibly the AWD's. Giving that important inner middle layer.
For the OPV's I think the 35-40mm should be sufficient for most situations. If we need more than that, then 57mm or 76mm should be considered. There are some capable anti air/antimissile rounds in 76mm. We already have 76mm guns.
It would be easier to up the main gun than try to mount 6+tons with good firing arcs somewhere else. If you really wanted a layer defence then 3t of 35mm millennium gun is going to be easier to mount on a small ship.
My preference is only guns on smaller ships like the OPV. Cheaper and more suitable.
Depending on the CMS adopted ExLS could be an option with its low platform impact and soft launch compatibility with RAM Block II, Seaceptor, Nulka, Griffon etc. A pair of triple quad packed launchers would be fairly easy to fit to any of the three short listed OPV options.