On playing Iron Saga Zero

Hello, I have several questions on the game and would appreciate any help. I have played the original some time ago but quitted just about when I unlocked Vassago Fist so I am not that clueless. I'm playing again to scratch my mech itch.

-Is the Zero version basically running from the beginning with events from the original trickling down the line? Can I use this to plan out pulls as f2p? Though I highly doubt it due to the amount of pilots and vehicles available really, and the overall non-existance of tier lists.

-Speaking of tierlists, can anyone give tips on what pilots and mechs to look for? My only notable build right now is Slokai with Nero which I heard should secure my PvE needs.

-I really like Slokai and Nero and is aiming for Retrofit as soon as possible. However, I was wondering if once retrofitted, do they still use the original mech version as duplicate fodder for upgrade?

-I am also planning to use special/collab units, the Watermelons and IceX for now, since they can easily be maxxed out without duplicate, are they good? Also I got a second IceX but I have no more use for it, should I just turn it to alloys?