I've been doing some fiddling with the Brinstar .rel file, thought I'd post my findings here.
Not sure what formatting I should post it as, so I hope you understand my references Acid
860 - 04 - 3880000E - Damage
850 - 0C - 38A0006E - Knockback
860 - 08 - 38C0005A - Angle
880 - 08 - 3900003C - Setting to 0 makes it so you are only hit once
And a more important find, you can change the effect type of the hitboxes.
870 - 04 - 38000005 - Fire Effect
This line makes the Acid hitbox apply the fire effect. You can change it for example to 3800000E which applies flower effect.