I think Simon will be useful but Dash will probably bail at the first sign of trouble (if he isn't actively siding with the forces of weirdness) and Mars will poke them with sticks and make it all worse.

Unicorn blood Mabel is awesome and you should totally get her, but i think a blood soaked coupon is more than a fair trade - you should get some cute accessories or something as well if you're giving precious haemoglobin to the gang of vampires that live in the mall.