Yes, you need to match the collett size to the tungsten size, they must match.
I'm also wondering why you would want to tig weld mild carbon steel with stainless steel filler rod? Yes, it will work but stainless steel filler tig rods are far more expensive than mild steel filler rod. 70s-2 filler is great for out of position work with a sluggish puddle, where 70s-6 has a more fluid puddle and excelent wetting action. Both fillers are very easy to work with and cost much less than stainless steel filler..