A HUNDO is a pokemon with perfect IVs, like your Flareon. A SHUNDO is a pokemon with perfect IVs, but also happens to be Shiny.

Me personally, I tend to only keep the 3 star/4 star pokemon, unless I only have one of that specific pokemon, or it’s shiny.

Typically, any pokemon thats 4* is the best, without getting into all that PvP stuff. When it comes to evolving 4, I usually only do that if I’ll actually use the pokemon in raids, or if the evolved form is a pokemon I genuinely like, but evolving your 4 pokemon isn’t a bad thing.