After some looking I could not find anything on Attachment cards limit. If I have a deck containing three .45 Pistols and three Sword of Y'ha-tallos; in theory could I attach all six of those cards to one Character card, lets say Overzealous Initiate?

Yes you can, there is no attachment limit. However, it's putting a lot of eggs in one basket - if anything happens to that character you're potentially losing a lot of cards at once. Most players prefer to spread them out to avoid that risk, although of course the situation may override such general concerns.

Been trying to improve my game in Core CoC. Playing around with different deck theories and experimenting. Thanks for all the help with rules, everyone!