Dont worry too much about the 400s, the reason they mix the 2 strokes with the bigger 4 strokes, is because the little 2s can hold their own with them.
Just take no unnessesary risks, and stay on the powerband.
I always have spare front sprockets in my kit, 12, 13, 14, and will swap them depending on track, conditions and competition.
I usually never mix chains and sprockets, but have been known to sacrifice a chain and or sprockets for a good placing.