Okay, I know you like the idea of a "jack of all trades" team, but people forget that the other half of that saying is "master of none." So as others have already said, it's best to use them in places that benefit your Pokémon. Likewise, while having a second megastone isn't the worst in the world (it does give you the option to pick which of the two goes mega to better respond to enemy teams), it forces you to think of both Pokémon as being both mega and not mega, looking at how their abilities and moves work for the build without going mega and after going mega. For example, if you mega with Blaziken, how does that affect Gardevoir? If you mega with Gardevoir, how does that affect Blaziken? Also keep in mind that whichever one doesn't mega effectively has no items for the entire battle.
Second, you already said you're not 100% sure about Crawdaunt and Kommo-o, so let's work with the four you have already and see where some holes can be filled. I'm assuming since you're using both mega and Z-crystals that this is a USUM team, so we'll work with what you have available there. Blaziken, Magnezone, Garchomp, and Gardevoir. That gives us Fire/Fighting, Steel/Electric, Ground/Dragon, and Psychic/Fairy. You have three types that are neutral or super effective to all of your team, being water, ground (with half the team weak to it, one of which having a four times weakness to it), and ghost. So it's best to pick your remaining two (or perhaps some abilities) to counter these.
Blaziken
Ability: Speed Boost
Nature: Jolly
Item: Wide Lens
EVs 252 ATK/252 Speed/4 HP
Moves
-Blaze Kick
-Sky Uppercut
-Brave Bird
-Bulk Up
Keeping with the moveset you selected, the Wide Lens is better than a megastone. You don't hit quite as hard, but considering half your moves have below 100 accuracy, it's better to boost that accuracy for them to guarantee consistent hits. You still have Speed Boost without going mega, so that's always good too.
Magnezone
Ability: Sturdy
Nature: Modest
Item: Weakness Policy/Aguav Berry (if Recycle)
EVs: 252 HP/252 SpAtk/4 SpDef
Moves
-Magnet Rise
-Thunderbolt
-Flash Cannon
-Signal Beam/Volt Switch/Recycle
The way you built your Magnezone was... problematic. You were trying to be a slow, heavy hitter using Gyro Ball, but Magnezone is a better special attacker than physical. Additionally, you had one move dedicated to guaranteed paralysis and another move with a potential for paralysis. Paralysis lowers the speed of the opponent, thus making Gyro Ball weaker. If you still want a good Gyro Ball user, I'd recommend Dhelmise, without any paralysis moves. For Magnezone, however, I elected for a tanky special attacker. Magnet Rise counters your greatest weakness in Ground attacks, giving you levitate for several turns. Flash Cannon and Thunderbolt are both powerful and accurate STAB moves. Signal Beam is coverage, while Volt Switch allows you to try and do damage while pivoting out of unideal matches.
Of the two, I lean toward Signal Beam as you're already slow, so don't want to waste taking a hit to run from something dangerous to you. Weakness policy means that if you do take a hit from a super effective Fire or Ground move, you get a boost to your offensive power, and can hit harder, while Sturdy ensures you'll survive the first hit. Another good option is Recycle with a good HP restore berry, such as an aguav berry if you want to run on being bulky.
Garchomp
Ability: Rough Skin
Nature: Adamant
Item: Assault Vest
EVs: 252 ATK/252 Speed/4 HP
Moves
-Earthquake
-Dragon Claw
-Stomping Tantrum
-Crunch
Your team isn't built around sandstorms, so both Sand Veil and Sandstorm are less than ideal to run with, so Rough Skin for added damage to physical attackers and assault vest for added bulk against ice beams. Earthquake and Dragon Claw are the highest power 100 accuracy moves of your STAB types. Stomping Tantrum replaces Rock Slide as it is same power, same coverage, and has 100 accuracy. Crunch rounds out as coverage for ghost and psychic enemies, especially since ghost is a weakness of Gardevoir.
Gardevoir
Ability: Trace
Nature: Timid
Item: Choice Specs/Gardevoirite
EVs: 252 SpAtk/252 Speed/4 HP
Moves:
-Moonblast
-Psychic/Psyshock
-Energy Ball
-Signal Beam/Hyper Voice
The problem with your existing set was that it had the Gardevoirite without any good normal type moves to benefit from Pixelate. As such, I've put Hyper Voice here if you choose to have this as your mega. That said, Gardevoir doesn't need to be a mega. You already picked out two great STAB moves, though Psyshock is a great alternative to Psychic for use against special walls, as it targets their lesser defense stat instead. Energy Ball is great coverage for dealing with Water, Ground, and Rock types, while Signal Beam addresses Psychic, Dark, and Grass types. I prefer running choice specs here over this as the mega to allow for hitting that much harder. But if you want this as the mega, we could look at an alternate 6th as opposed to the mega I list below.
That leaves the remaining two. You have three options here for a mega, but none of them need them, so let's build around what's remaining and include both our mega and our Z-crystal user.
Mimikyu
Ability: Disguise
Nature: Adamant/Jolly
Item: Mimikium-Z
EVs: 252 Atk/252 Speed/4 HP
Moves
-Play Rough
-Shadow Claw/Shadow Sneak
-Drain Punch
-Swords Dance
Pretty standard Mimikyu build. Shadow Claw/Sneak is choice between power and priority, but both are STAB. Drain Punch is great coverage for Steel, Ice, Rock, Dark, and of course Normal types, as well as providing some health recovery after disguise breaks. Play Rough is not only Fairy STAB and the only physical Fairy Move prior to SWSH, but it works with your unique Z-crystal to become "Let's Snuggle Forever" which jumps from 90 base power to 190. Even without a Swords Dance up, not much is surviving that. In fact, Swords Dance can be replaced with Shadow Sneak so that there's no need to choose between the two, though it's still recommended.
Blastoise
Ability: Rain Dish (Mega Launcher after Mega Evolving)
Nature: Modest
Item: Blastoisite
EVs: 252 SpAtk/126 Def/126 SpDef
Moves:
- Aura Sphere
-Dark Pulse
-Scald/Surf
-Ice Beam
Aura Sphere and Dark Pulse are great coverage moves that get their power boosted by half by Mega Launcher, effectively being STAB in power. Scald/Surf are STAB with a difference of 10 base power with the choice of burning or more power. Scald should usually be the pick. Finally, Ice Beam because every team should have someone with it for all those pesky Flying, Ground, Dragon, and Grass types. With this moveset, you have super effective coverage for 11 of the 18 types on one rather bulky mega.
