the mini battery is different it goes under the seat, this new redesign makes the mini battery the same
maybe on the city and rover they were...
but i would confirm that with rad power before buying
for resale if i was buying your bike a year or 2 from now the interchangeable battery would be a big deal to me
just looked at their website real quick, the battery is 2 or 2.6 ah bigger on the 2018, think the 17 is 11.4
this would be a big selling point for me, the more ah the better
cant help you on the answer of not using a third party battery, that would seem strange but i havent had any issues with my rad batteries and would seriously consider going back with their brand if i needed a new one- but maybe not if the ah were a lot higher on a good aftermarket...
sure they can answer that question for you
good luck, sure you will be happy with either