Flareon would be better if it actually had more physical moves. Seriously, the only physical moves that aren't normal-type are Superpower, Bite, Dig (lol) and Iron Tail. Also, apparently every Eeveelution that isn't Jolteon has to be as slow as Muk.
I read a fan written analysis regarding Paras and Parasect. Supposedly the fungal infection and the Paras have something is closer to symbiotic relationship rather than actual parasitism. The fungus cannot infect or spread to other Pokemon, even species that are believed to be related to the Paras. On the other hand, Paras relies on its spores to ward off predators and the fungus actively provides its host with protection, in exchange for the Paras doing... literally everything else. Essentially, neither cannot live without the other and I'm convinced they're pretty much a single entity because of this.
Granted, the fungus lacks sentience and acts purely on its primal urges to feed on tree roots, so they are still technically a zombie in that sense. But this analysis did speculate that it was possible to suppress the infection with antidotes and I assume this would also prevent the fungus from completely overriding the Paras' nervous system, allowing the bug to keep its individual personality intact rather than devolve into a husk upon its evolution. So, Spora not being a zombie isn't far-fetched at all.
But honestly, with all that taken into account, I kind of wonder if Spora has her own internal battles against the fungus' presence which explains her socially withdrawn nature. Of course, if Spora went down this route, I feel like Cornered would have taken a much darker turn.
On the topic of Flareon, since it has Guts as a hidden ability you throw a toxic orb on it and give it Facade since it can learn that, give it flare blitz for damage, flame charge so it can get it's speed up, and then superpower. Not the best but I think it works pretty well, at least with flareons current move pool.
On the topic of the parasite being symbiotic, that's entirely bull. You might think the mushroom is offering protection, but lets think about this in actual game terms. It's giving a bug type pokemon the grass type. Grass which is tied for the type with the most weaknesses as Rock, with in combination with Dry Skin makes Paras and Parasect the only pokemon in existence to be able to have a 5x weakness to fire.
Now let's take that into the idea of stats. Take that mushroom of Parasect's back and we'd likely see a reduction to both physical and special defense, but see an increase to health, attack, and speed, since the mushroom isn't stealing it's nutrients and weighing it down. The other thing is that in most dex entries, aside from stating the mushroom controls it, it otherwise states Parasect burrows underground to drain the nutrients from tree's, which leads me to believe Parasect could be a Bug/Ground type if it wasn't for the mushroom.
Now the only saving grace Parasect has in a competitive sense is that it's got Spore. Which the mushroom provides, unfortunately every other pokemon that has Spore, (namely Breloom) is better at using it, so the one thing the mushroom gives Parasect that makes it useful is also hindered by said mushroom.
So in short the Mushroom gives it a typing that makes it weak to more types of attacks as well as possibly keeping it from a different much better type. Since it drains parasect it likely reduces the Health, Attack, and Speed stats, in exchange for defense stats that don't matter with the added weaknesses. And a plethora of grass moves it really doesn't take advantage of, and the ones it does take advantage of, Spore, are rendered moot, and Seed Bomb, the only physical grass attack Parasect gets while being a pokemon with an abysmal Special Attack.
So yeah, I don't think the mushroom is symbiotic.