I’ve never been a competitive player, so I’m still learning as I play, but I’m working on my team. I don’t have completed move sets yet, so right now I’m mostly focusing on type advantages, I guess.
Right now I have: Flygon Haxorus Delta Gardevoir Swampert Delta Venusaur Blaziken
Do I swap Flygon with Gengar to cover fairy? I’m also considering a Heidregon to swap with Haxorus. Thoughts?
Edit: Changed Venusaur to delta
Well you’ve got quite a few pokemon that can go mega. Which one do you think will be your primary mega?
i have tyranitar , Delta Charizard, Delta Gallade , Gyrados , Snorlax and dont have any last pokemon defeat 4 gyms! 
thats my team
Mega Delta Venusaur doesn’t seem like a bad pick. Hubris is a nice ability, granting a boost to special attack with each defeated opponent.
Generally speaking, Flygon isn’t viewed as a competitive pokemon, at least when compared to other dragons like Garchomp. Statwise it’s well rounded and has a diverse move pool that allows it to function as an attacker or a special attacker, with a few good support options. But there are generally better options. That said, I’m rather partial to Flygon, and Insurgence gave it a mega that has an ability that boosts the power of sound based moves. Base stat wise, Flygon is superior to Gengar in all but it’s speed and special attack. But Gengar has an impressive 130 special attack and a 110 speed.
Gengar is conversely regarded as a really good pokemon competitively. 130 special attack is superb, and it’s got a move pool to make use of it. Not only that, it’s got the speed to sweep. But it’s a glass cannon. It can’t take too many hits. Ideally it’s fast enough to sweep anything before they can hit it back but it’s by no means immortal. It will hit fairies hard. But it also has to worry about dark type moves hitting it hard. Given the New Moon weather condition in Insurgence and people talking about making New Moon teams, expect for Gengar to take a few dark hits.
If you want more information, serebii has some good reviews of both Flygon and Gengar.
I don’t really see why you would need dragon types to begin with, let alone two, since delta venusaur and delta gardevoir cover dragon types well enough. I’d say get rid of haxorus and replace it with something like aegislash to cover fairy and psychic types and if you really want a dragon type get rid of flygon and replace it with something that strikes harder like garchomp. Since you’ll be having a ground type if you do that, it wouldn’t be a bad idea to replace swampert with a protean greninja as well.