It doesn't matter since you can only link with one Eevee...
But you are pretty lucky if you found Eevee swarm o.O I haven't found any Eevee swarms beside the one I made by using Password.
I've only done the Password one. I just call it a "swarm" because it's basically like the Pokémon swarms in the main series.
At any rate I finally got the evolution stones to get Vaporeon, Flareon, and Jolteon, but I think I'll keep my main Eevee (the one my player character uses) as is for now because I've read if you wait till you get to Nixtorm, where you can make it evolve into Glaceon, that will help a TON with the final battle against Nobunaga. (Probably because that's the Dragon-type kingdom, and Dragon-types are weak to Ice).
@WillPowerPedro: I don't believe Conquest shares some similarities with Ranger... Yeah, there are some. Like you have limited amount of Pokémon you can have with you but in contrast to Ranger you can actually level your Pokémon and make them stronger.
And not only Pokémon but also people can join. =)
I really think you should give it a chance :)
The strongest similarity Conquest has to Ranger is in the Links; this is very similar to the "befriend-rather-than-catch" method of getting Pokémon in Ranger. Also, there is the Pokémon limit as you said, though technically that is in the main series too; however, the prompt to let a Pokémon go if you have too many is just like Ranger. Also the gameplay is similar in Conquest to
Shadows of Almia in that you have a main storyline with missions to perform, but what you do with the rest of your time is up to you.
You cannot miss a chance since you can do all the missions over and over.
But onto your question.
You must fulfill conditions needed for transformations.
You can find them on serebii or bulbapedia.
Actually only Warlords can evolve. Normal Warriors can't. Oichi is the exception to this; she's a Warrior but counts as a Warlord for some weird reason, though she only has 2 levels like most of the Warlords (only the Hero and Hideyoshi get 3 levels), so once she evolves to Level 2, she doesn't evolve any more.
As for the conditions, they're basically the same for every Warlord: reach a certain link percentage with their Perfect Link Pokémon or one of its evolutions. The one exception to this is the Hero - in order to reach Level 3, the Hero must link with all the Eeveelutions.
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I've attached the walkthrough I talked about as it currently stands to this post. Could someone please read it and tell me if it makes sense? I've never written a game walkthrough before.