they're pretty useless for anything other than a dex filler aren't they? Or is it worth investing stardust and candies in to him? Are they ever good in gyms/defending/fighting?
You're right; they're pretty useless. I wouldn't invest any stardust or candies in them; it would just be a waste. I have never used mine to fight or defend.
Am I right in thinking skamory will evolve in a future gen?
Nope; Skarmory never evolves.
DH says there are a couple of charizards holding local gyms - what does anyone reckon I should use to kick It's ass?
Charizard is fire/flying, so your best bet is rock - it's super effective against both fire and flying.
High CP Pokemon like Golem or Rhydon that have Stone Edge are the best to use, but don't use either Pokemon if they have ground moves - you need rock.
Water and electric are effective too, so any high CP Pokemon that have water or electric moves should be your go-to after rock.