That Fox, if it is tight and on face, and the wood in the head isn't damaged or oil soaked too badly, should handle those loads fine. Imperative also is to make certain the bores have not been honed beyond a very few thousandths, and that there is no bad pitting in the chambers or near the breech end of the barrels.
I hunt ducks with Foxes and hand loaded bismuth every year, and my load is hotter than the RSTs, but that's my call, determined by the condition of my guns and the pressures of the load. Personally, I like a heavier gun that the one you're looking at, for most waterfowling, but that's another personal thing. All my vintage duck guns weigh from 8/2 to 9/6. The old 30" BSS, while not a vintage gun IMO, is lighter at 7/11. All of these have used 1 1/4 oz. duck loads with no issues, other than having to shim the hook on the BSS from so much use.
Merry CHRISTmas to you, too.
SRH
