In my experience Swellow is just like Pidgeot. Good early on but lacks the tactical punch nearer the end.
Aron however is awesome. A real tank he can take alot of physical punishment and learn some interesting attacks (including Aerial Ace) to take out Fighting types that go against it.
Shroomish ive never trained so i wouldnt know though if your going to have a Lombre/Ludicolo theres no real need for it in the team.