welcome- and you will probably get lots of different answers because most things are a personal choice- there are no hard-fast rules when it comes to quilting
first the thread- visit the Superior Threads web site. they have a free chart which you can print off that tells you about all the different types/weights of threads- what they are best used for and what needles to use with them- it is an invaluable resourse.
i use Aurafil alot- some cotton threads and some poly blends and even silks for piecing- depends on the project- everyone has their preferences- as long as the thread is strong- the right color-(for piecing i normally use a neutral- light gray or cream for light fabrics, darker gray-taupe for darker fabrics) some people like to match colors with their fabrics.
second- the fabric companies who market pre-cuts will tell you they do not expect you to pre-wash them before using-
if you are worried about bleeding (usually only a real concern if reds, or handdyes) you can do a 'color test' on individual pieces- wet a white cloth or paper towel and rub it on the 'questionable' fabric- if color comes off on the cloth you have a bleeder- if none does you are safe-
if you choose to pre-wash your pre-cuts there is lots of advice how to go about that- by hand in the sink, in a mesh laundry bag, ect. if you choose to not prewash you can always toss a 'color-catcher' into the first wash of the finished project for (insurance) incase anything does run a bit