Hi, we've been playing colossal since last December. I've just been re-reading the rules and I think we've been doing it just a tad wrong when drawing cards.
What we've been doing is instantly discarding cards that we pick up that are for creatures that have been eliminated. No where does it say you can do so, so I guess you have to keep them? You can only discard cards for eliminated creatures in the discard phase, and then only three.