I know that SHrink goes off of original atk for monsters like Injection and Trooper, but what about for ? monsters like Gren Maju? It's original atk is considered it's current isn't it or would shrink do nothing to that kind of monster?

if an effect determines an original attack, like chimeratech, then shrink will halve it. however, gren maju da eiza's effect does not determine an ORIGINAL attack, just current one. so shrink would not work on his effect.

from netrep

based on that his original attack is treated as 0. his effect just determines his current attack.

So with Chimera it would actually half it since it is determined upon summon and can't change, but something that does constantly change it would do nothing to.