I highly doubt that you can get it up in the air under 250g.. Ill go and weight mine later but iirc my weight without battery was over 250g.
Should be doable.
My Mighty Mini Zero was 226 grams with an F pack motor and 500mah 2s. It would still be under 250 grams with an 800mah 2s.
A 3s battery is quite a bit heavier. With an 850mah 3s battery I came in at 269 grams.
Now, the Zero seems to be about 20 grams heavier empty than both the corsair and the mustang. Also, keep in mind that the above weights are with paint, Ross foamboard (a bit heavier than dollar tree foam), and the landing gear as pictured here:
Shouldn't be a problem getting under 250 grams with the mustang or corsair & 3s battery if you're careful with your hot glue and tape applications. If you're really concerned about it, remove paper from the inside of the fuselage when you build it. You won't lose a lot of strength that way, and you'll save quite a few grams.