If I capture a fighter and end up retaining his equipment and it is not standard equipment for my gang, like a Redemptionist Eviservator or Scaly Discus can I give them to my fighters to use?
No. Although the Capture Serious Injury Result on ORB page 84 states that the captors keep the equipment any equipment that your cannot inherently use, like a Dealque gang getting a Scaly Discus, the equipment is either stashed to trade at a later time or sold at the trading post using normal trading rules. If a weapon has a similar entry on your list, say a Scavy Scattergun and a Ratskin Blunderbuss you can still use it since they have the same basic stat lines, just different names. Basically, treat all non standard equipment as unique equipment. This means that if you cannot already use it you must stash it or sell it. The only reason that stashing the item is included is that you may be able to trade with that gang type at a later time and may want to hold onto it, but you will never be able to use it.