TB most of the shishitos I've had do not approach jalapeno heat much less serranos. Next time you go to Edoko please grab me out some seeds of that pepper and we'll trade for em.
I toss mine in oil with some onions and broil/bake until they just start to blister. Little salt and/or garlic is fine too. Maybe 1 in 10-12 has about the heat of a lower end jalapeno.