Buri: I prefer special attack Lucario myself since it can use vacuum wave instead of goddamn bullet punch.
Mageziya:
To be "good" at Pokemon, three things you must master:
1. Luck
2. Knowledge
3. Mindgames
This will be about mindgames. The whole purpose of fielding a team of multiple mega evolutions is about fucking with your opponent. He won't know which one will be the mega. It could be the Gyrados. It could be the Scizor. In your case, it can be any of them at any time.
However, let's look at this a little closer. If you actually give all of them mega stones... they inevitably lose actual items. This is actually a massive drawback. A Garchomp with life orb for example is quite literally 30% stronger than a Garchomp with nothing. In fact, a Garchomp with life orb actually hits harder than Mega Garchomp (but is less tanky). Similarly, a Scizor with Choice Scarf actually is 50% faster than a Scizor with nothing. Etc. etc.
Furthermore, not all mega evolutions are created equal. For example, look at Mega Garchomp. It's freaking useless. No one would mega evolve it unless they were really dumb. Mega Alakazam is another one that is quite definitely weaker than regular Alakazam. Many of the valued mega evolutions are actually only liked because of certain niches that they can fill. For example, Mega Charizard Y is probably the best weather generating pokemon right now. Mega Gengar is one of the best spin blockers and Mega Gyrados can switch typing and avoid stuff that would normally kill it.
So if the opponent sees your team of mega evolutionary potentials. With a little guesswork he can probably guess which one will be a mega evolution. For example, why would you field a Kangaskhan or a Mawile unless you were going to mega evolve them? They're quite useless in their normal form. Similarly, if I see Charizard and Venusaur in the same team, I know Charizard is going to be Mega Charizard Y.
So it depends on how well you can play the mindgame.