I would go with Absol, as you already have some very bulky Pokemon on your team, and in-game, it's all about powering through your opponent! Absol has a large movepool and, unlike Dusknoir, can set up a Swords Dance and sweep. Another reason I would go with Absol is because to get a Dusknoir you need to trade with it holding a reaper cloth and so on - it's just more hassle for a Dusknoir, and by the time you've got it, the journey's almost over. With an Absol you can catch it and use it straight off the bat.
By the way, I fell into the mistake of wanting to use Gallade for my ORAS playthrough as well. I caught Raltz, evolved it to a Kirlia... and it just stayed a weakling Kirlia whilst all my other Pokemon evolved further and became much stronger. This is because the earliest opportunity there is to evolve Kirlia into a Gallade is on Victory Road, after you defeat Wally. The journey was practically already over. If youhave a Dawn Stone ready and waiting, then you go for a Gallade, but if not then I would consider using a different Pokemon altogether.
I would use Azumarill, as it is a nice replacement to Gallade. I has a fast experience growth rate, can learn multiple HMs (if you want it to, as your team is currently lacking in HMs - waterfall and strength are good too you know!) and is generally more bulky with a much better ability, that you should use for an all-out attacker. Again this is only if you want to make the switch from Gallade to Azumarill, but I would.
As for the moves, I would say that you should replace Ice Beam and Dazzling Gleam for Toxic and Protect on Tentacruel, as Tentacruel is a more bulky Pokemon that should stall out the opponent, and be the ultimate Slaking counter. Other than that I think your moves are perfectly fine for an in-game team!!
Hope I helped!