are they neccassary and can u use them on ant type of seat or just the one it says. 
Also can u use a square frame such as a torker frame and put it on a sun uni?
handles are not necassary but they are a pain to ride with out.
Some people like them some people don’t. I personally don’t care.
It depends, If you do a lot of seat-in stuff, your gonna want a handle. If you are a SIF, a handle isn’t that necessary, but I would still have one.
As for the other question, do you mean putting the frame onto the wheelset?
like taking the frame of the sun uni and putting on a a square frame so yeah putting it on the wheel set.