I'd check with Specialty Alloys (this is not an advocacy-there are others). As you probably know you're going to wanna emphasize arc stabilizers especially in an environment of cladding (though to my usage cladding is sort of a misnomer in that the welding operation is generally called overlay-though strip cladding, a welding operation is common terminology) where you wanna miminize dilution either by low currents, fast travel speed, or playing with the Power Waves avaialble balances.
Companies like Specialty Alloys have been selling fluxes for overlays since the Himalayas were a barren plain.
Checked some notes, been awhile. NT-100 is the one recommended for Alloy 625 overlays. Make sure though they understand you are messin with the Power Wave and how you plan to manipulate the arc.