also does it synergize well
This is for the metagame/battle subway
To answer your sub-question, sort of. Their types vary and on one team there will likely be a type to cover everything excepting Dragons (Glaceon is a rare sight). However their lack of dual types means they loss some important types meaning they can be unsuccessful competitively.
No. They have great synergy, but just no. There are way better Pokemon to replace it. there are only like 4 great eevee Pokemon and they are jolteon vaporeon espeon and umbreon. Leafeon is pretty good but there are way better tanks. I would suggest using a regualar team. They get outclassed for exampe flareon is supposed to be a physical attac right? It has a poor physical movepool and other Pokemon can outclass it. take heatran for exampe. It isn't a physical attacker, but it has high special defense ike flareon and high attacing stats. Wouldn't you rather use heatran than flareon?