You can weld stainless with mild steel filler but you can't weld mild steel with stainless. Whom ever told you this lied to you. You can certainly weld mild steel with stainless. In the boilermaker field, doing xray welds all the time, stainless is known as "magic wire" when tigging in ro...

Just running stringers, try to not let the puddle get any wider than 1.5 times your rod diameter (flux included).
A stringer for a 1/8" 7018, after deslagging, should be 1/4"-5/16" wide.

getting i suppose 4ish inches or a little more of weld per electrode (3.2mm (1/8inch) electrode at ~120 amps) With a 1/8" 7018, if running simple stringer beads, you should be getting about 10-12 inches per rod. Anything shorter, talking stringers, is to slow travel speed. I'm also talking fla...