Yeah, Mega Mawile would be a good fit for A+. It's got great bulk, phenomenal typing, absurd offensive prowess, great coverage, powerful priority, a boosting move. It's got absolutely everything it needs in order to succeed, and its coverage makes it nigh uncounterable until you've figured out exactly what it's running. Of course, it's pretty much always going to have Play Rough and Sucker Punch, but from there, whether or not Skarmory, Heatran, Mega Venusaur, Rotom-W, etc. can check it depends entirely on what kind of Mawile build you're running. Jolly Mega Mawile can outspeed min Speed Rotom-W and can OHKO it at +2 with Play Rough. Mega Venusaur gets smacked by Iron Head, Skarmory by Fire Fang, Heatran by Knock Off and Brick Break. Only Mega Venusaur out of that entire list wants to take a Focus Punch. It's just a really phenomenal Pokemon, and it deserves to be in A+.
Gourgeist-Super should probably drop down to C+, and Gourgeist-Small even further down (don't really care where, I've never wanted to use it). It's a good Pokemon, but even though I never thought I'd say this, Trevenant is usually better. While Gourgeist has a lot more physical bulk Trevenant has a bit more special bulk and a ton more longevity. Gourgeist has to rely solely on Leech Seed and Leftovers for survivability, which is generally fine early on in the match, but really weighs down on it as the match progresses. Trevenant, on the other hand, is capable of either being a phenomenal status absorber with reliable recovery in Lum Rest or just won't die thanks to Sitrus Harvest and Leech Seed. This makes Trevenant much better equipped to take on Rotom-W, Mega Venusaur (can burn it, fearing only the generally rare Knock Off and doesn't care much about potential Poison from Sludge Bomb), Landorus (strongest move gets a 3HKO without factoring Sitrus and/or Leech Seed), Thundurus (LO HP Ice gets a 3HKO without factoring Sitrus and/or Leech Seed), Keldeo (Specs HP Ice has a 3% chance to 2HKO without factoring Sitrus), and even Mega Gardevoir (it can switch into Hyper Voice with Sitrus once and proceed to 2HKO with Horn Leech). Grass/Ghost typing is better for checking special attackers for the most part, and Trevenant doesn't get worn down nearly as easily throughout the match.
Barbaracle is bad, and so is Cloyster. It's not B-rank at all lol. Both are still outsped by the most common Choice Scarf user in the tier (Genesect) if they run +Atk natures, are incredibly vulnerable to priority, pretty much have to kill or be killed, don't find many opportunities to set up at all, and should remain unranked. Hell, Barbaracle can't even switch into Talonflame and set up because:
252+ Atk Choice Band Talonflame Brave Bird vs. 4 HP / 0 Def Barbaracle: 104-123 (36.3 - 43%) -- guaranteed 3HKO after Stealth Rock
Switches in, takes ~40. If you Smash as the Talonflame user switches out:
252+ Atk Choice Band Talonflame Brave Bird vs. -1 4 HP / 0 Def Barbaracle: 156-184 (54.5 - 64.3%) -- guaranteed 2HKO after Stealth Rock
With Stealth Rock, that's a guaranteed KO. Without it, there's a solid chance of KOing.