Got my feet wet, not sure how to build my core though

Hey all, just got back into the game after I realized a few things, checked out the guides on the side and I am still a bit confused despite having more info at my fingertips.

I kinda see why a 2 core is better than a 4 core, but fun wise, for me, I think 4 core would be better.

I currently am rocking: Druid +200, Naga +200, Alchemist +200, Drummer +100, Succubus +100, DK +100, Fairy, Unicorn Knight, Raptor Rider, Hot blooded, Black Magic Wizard, Cavalary Knight all +100

In the Time Shop: Raptor Rider, Wold Rider, Hands of Death, Sword Dancer, Gunner, Wind Mage, Great Hammer Unit, High Elf Archer, Hammer Knight, Musketeer, Frost Mage and Elf Archer.

Current comp gets me to about stage 500. I have read to not have more than 55% of your medals be spent on supports, but I am not sure which should be my core 4.

I have read these two posts

https://www.reddit.com/r/EndlessFrontier/comments/5cjebq/guide_to_constructing_your_core/

https://www.reddit.com/r/EndlessFrontier/comments/5cc6m9/guide_to_allocating_support_medals/

Shit I just might have got why core 2 is better.

Soooo with that realized, with my current setup, should I have my druid and naga be my cores? (at least for now?)

Do cores have to be same race? (Also are races the same as tribes? If not what are tribes?)

Lastly also saw a tier list (I cant seem to find it now) that listed units from top to bottom in ranks of S A B C D. Is S for support or higher than A in importance?

Sorry for all the questions and thanks for any insight and input :)