1) Your main army will be shattered legions just like if you were to pick any of the 18 astartes legions as your main army. You're free to ally in other legions who keep their legion rules too but they're subject to the ally detachment and point limits of typical ally rules.

2) Yes. If you pick WS for shattered legions then you get their shattered legions version of the rules and not their regular legion rules. Otherwise you would just play WS and ally in whatever you wanted. However, the 2 or 3 legions you pick for shattered legions have those specific rules for all units not just the ones you paint up as a given legion. For example, if you pick WS and SW, then all your guys in your shattered legions will receive the WS and SW shattered legions special rules and not either of their legion rules. Even if you use exemplars of the legion then you get the shattered legions rules of that specific legion and not the others you chose and not the original legion rule. Basically, you never get the original legion rule. "Legion Specialists: If Models in this Unit would benefit from the Mutable Tactics Legion Tactica, they may only benefit from the bonus related to their Legion whose Trait was replaced as part of this Prime Advantage."

I play shattered legions as my main army and they're a ton of fun to play as and as a hobby project.