While researching shooting roles, it looks like there are a lot of minor changes in Savage Worlds Deluxe around shooting, like double shotgun rules, fanning/rapid attack etc. Should these be considered errata and trumph the Deadlands rules, or is Deadlands Reloaded (specific: The Flood campaign) balanced/designed for the previous editions and one should stick to an older ruleset?
The latest version of Deadlands Reloaded was originally published before the Deluxe version of the core Savage Worlds rules was released. Subsequent to this, Pinnacle Games released a pdf document that can be found here, which details any rules alterations required to make Deadlands Reloaded compatible with the Deluxe version of the rules.
This means that there is no reason why you should not use the Deluxe version of the rules with Deadlands Reloaded, including all of the rules alterations that in my opinion, make for a better, more balanced game.
Also, it is worth noting that Savage Worlds' general principle is that you should use rules in the core rulebook unless they are explicitly overruled by the setting book you are running.