ZOMBIES!!! 3.5 - NOT DEAD YET!
Do I mix the expansion set cards with the core set cards?
Yes, but as an alternate rule you could place them separately and decide which deck to draw from.
Source: Kerry June 16, 2005
This card is played when an opponent plays a card. Does it have to be played simultaneously or can it be played on a weapon that's already in play?
It can be played anytime after the card is in play, so yes, you can play it on a card that is on the table already.
Source: Kerry January 26, 2007