Milotic: Your reasoning is sound for Hyper Beam, but it's not worth it. Not only does it leave you vulnerable for one turn, but Milotic can't use Hyper Beam effectively with its awful Atk stat. You'd be better off with another SAtk move in my opinion. Like Dive or Waterfall for utility (outside of battle) or Hydro Pump. Reflect or Light Screen would be great, too, but I'm not entirely sure Milotic can learn it.
Bellossom: Sunny Day and Solarbeam sounds like a good idea. I would use a skill set like this: Giga Drain/Solarbeam/Sunny Day/Moonlight. Giga Drain isn't necessary, you just need an attack move for when Sunny Day is down (as a backup). The reason I kept Moonlight is because healing moves are really useful for saving potions. Not only that, but Moonlight in particular has good synergism with Sunny Day (which Bellossom's moveset revolves around). The reason I would choose to ditch Sleep Powder is personal preference. I simply can't rely on it because of its accuracy. There's not really any room in the "new" moveset, but if you must have it, you would replace Moonlight.
Gardevoir: Confusion and Imprison definitely need to be replaced. Imprison is just way too uncertain, to me. Even if it does work, there's not much another Psychic Pokemon can do to Gardevoir regardless. So I would replace those two moves with Thunderbolt or Shockwave, and Reflect. Thunderbolt (or any move, really) accomplishes the same purpose of finishing off a low-HP target without using Psychic's PP, but more importantly, it give Gardevoir more type coverage, allowing it to hit super-effectively against Pokemon weak to Electric (which, of course, helps it do its job). Shockwave can be used, too, if you don't have Thunderbolt. Reflect not only helps the team, but its main purpose is to let Gardevoir set up Calm Mind with ease. The whole moveset should revolve around Calm Mind, and then destroying anything that gets in your way with CM-boosted Psychics and Thunderbolts. Calm Mind, and any stat-boosting move like it, completely trivializes the game, in my opinion.
Blaziken: The moveset I would suggest is, again, going to rely on a stat-boosting move. Bulk-Up/Earthquake/ Brick Break or Sky Uppercut/Aerial Ace. Like I said earlier, I think stat-boosting moves just let you completely cheese the game, so this set is what I would do. But if you don't have the TM or Heart Scale to teach it Bulk-Up, then a mixed sweeper build works just as well. So the second option would be the same as before, but replace Bulk-Up with Blaze Kick, Fire Blast, Flamethrower, or Overheat. I'm pretty sure all Fire-typed attacks are Special, so Blaze Kick still works for hitting Pokemon with low SDef. Since Overheat would be your only Special attack, it works well with this set since it only gimps itself when you use it. If you don't have Flamethrower, then Fire Blast is reliable enough (in my opinion) and extremely powerful.
Absol and Skarmory seem like solid additions to the team; Skarmory covers your Fly'er, and Absol is really good (but Absol is my favorite Pokemon, so I'm pretty biased haha).
These are my suggestions based off my opinion, so you don't have to follow them perfectly. You can roll with pretty much anything and still beat the main game.