Hmm lots of questions, not sure if i really know the answer to them all.
It all depends on the presentation your looking for and what you feel happy fishing with. If you want a poped up presentation or a presentation where the baits just standing up then go for pop ups. But if you looking for pretty much just a doubled up bait sitting on the bottom then your normal boilies will do fine.
In terms of rigs, if your using 15mm boilies and you want to snowman two of them then your hairs going to need to be 15mm + 15mm + 10mm (i leave 10mm gap between bait and hook, just personal preference, some leave 5mm), so your looking for quite a long hair which you will find hard to buy ready made.
So yes you will need to start making your own rigs, and is something you really should learn to improve your fishing.
In my experience everyone have their own ways of tying rigs and what rigs they prefer to use. If I was you id purchase a book with all the well known rigs in. This way you can browse them all, choose your favourite set up, make ulterations depending on your preference and fishing senario and then go from there.
But 99% of the time Ive found simplicity is best.
P.s
Korda Extenda stops can be very helpful if you tie your hair too short.