s&s is a good coupler but there are other diy solutions. back in the late 80's, somebody was even selling stainless pipe couplings which mimicked the s&s. Check the Ritchey breakaway frame where the downtube inserts into a male tube at the bottom bracket and the toptube is coupled to the seattube via seatpost.
The 20" wheel will make packing easier but not everybody likes the look.
I personally ride a 20" homebuilt frame where the rear triangle rotates towards the front triangle.
if you're building it, you can do what tickles your fancy
