You're fortunate - your level 50 champs are all worthwhile. Not necessarily to level to 60 yet, but likely eventually will be.

In terms of your 40s - I would recommend keeping all your epics. Even if you don't have need for them now, you may eventually, and in the interim they can be used as faction guardians.

For the rares and below - there's two schools of thought. Personally I like keeping a single copy of all rares and above, and I also have my rare faction guardians filled. But this comes at the cost of vault space and potential food. If you choose to go with the latter route, save all rares that are necessary for fusions until you have completed those fusions, and beyond that, only save the rares that have specific use for your current situation. These include, but are not limited to,

• Coldheart (spider, FK, DT)
• Metalshaper (DT Scarab)
• Reliquary Tender (DT Nether Spider)
• Master Butcher (DT Bommal)
• Paragon (various uses)
• Pain Keeper (Unkillable CB Team)
• Renegade (Unkillable CB Team, dungeon speed farming)

Because I have Kalvalax and Venomage can or should they replace Frozen Banshee in Clan Boss?

I thought as far as rares go that I would keep Apothacary and those you listed although I always thought Master Butcher was trash

I think there is a third way in the middle between the two ways krama described: in addition to the rares for general content, krama mentioned some examples, keep rares that are good for faction wars. You don't have to level or equip them right now, just keep them for later use in your vault. At some point later on in the game you probably want to do the faction wars, and some rares have abilities that are, well ... rare inside of their faction. 

For example Fleshmonger. There are exactly two champs from his faction Skinwalkers that can put the big decrease def debuff on all enemies: the void legendary Khoronar and Fleshmonger.

Another example is Geargrinder. He is a reviver. You don't need him for general content, because you allready have Scyl of the Draces, who is a far better champ with revive ability. But in his faction Ogryn Tribes, he is the only real reviver. The epic champ Uugo has a revive ability, but it's conditional and works only, if all other champs in the team are dead, not to mention that you don't have Uugo at your account. So Geargrinder could be usefull for your Ogryn faction war team.

Typical abilities you want to have in faction wars are decrease def on all enemies, heal, revive and turnmeter reduce.

P.S. 

 I thought as far as rares go that I would keep Apothacary and those you
listed although I always thought Master Butcher was trash 

Master Butcher is not only great for Bommal, but also for Magma Dragon. Master Butcher has a two turn provoke with 100% chance to land on his A2. 

is that pretty much the list of gotta keep rares except maybe Bellower , Royalguard and Fellhound?

Royal Guard is epic and if you ever are lucky enough to pull this champ, definetly max him out as soon as possible.

Beside that, it's nearly impossible to make a list of all rares worth keeping. This list would depend heavily on your alternatives. For example my starter champ was Athel, but after nearly two years of playing I built a second champ from the starters, Elhain. The reason for that was, that my High Elves faction war team needed ages to complete a crypt, the team had no dedicated damage dealer. I could beat stage 21 allready, but it took 30 minutes of manually playing. If you have Royal Guard, or Royal Huntsman, or any other great damage dealer from the High Elves, you don't need Elhain anymore. So does Elhain belong on the list of rares worth keeping? There is no clear yes or no to this question. For some rares, the answer is just "yes", no doubt. For others it's "maybe".

Thanks for putting it in perspective as I wasnt even thinking about Faction wars teams