5 degree delta with a 20x20? May want to double check that, with the 26-99 on speed 1 with your piping arrangement that's only about 15,000 BTU, and a 20x20 in a standard furnace will push out easily double that with your 6-7 GPM flow rate. Plus, a plate will suck the life out of the boiler water in a hurry. With 2 showers running simultaneously can get over 150,000 BTU which, at 7 GPM will pull the boiler water down 40-45 degrees alone. With the 26-99 on high and the primary piped as I previously described, you would be closer to 8 or 9 GPM with your short run of pipe which would pull that delta down considerably. As RSI pointed out, there is no sense running the plate on a secondary loop as you'd need to run the circ 24/7 anyway or add a flow switch to activate the pump on any DHW call. Unless you have heat creep issues, there's no benefit to running the 20x20 on a secondary either. Pressure drop on either is hardly enough to notice on the pump/flow rate, so that is of almost no concern. You can still use the manifold if you really want to and just valve off the other ports for future additions