I only can answer these two above....
(1) You can start cardio and sit-ups and other excercise at the
same time.
(2) It is impossible to answer "how much is enough" unless I know
you personally....and if anyone can answer this specifically,
please don't believe them. I need to know your lifestyle, how much
you can take, how you feel during the exercise, etc. In other
words, for the first time, I need to run with you....and gauge from
there.
(3) Skipping is cardio, but running is better. Better still,
combine the two. No single exercise is enough....thats why there
are so many types of exercises invented - they all complement each
other. The problem with skipping is that many people get
discouraged.....and for beginners, there is really not much cardio
because they spend most of the time trying to start the rope up
again (after it gets caught between their legs).
The idea of slimming (for most of us anyway) is to appear more
attractive to others, especially the opposite sex. I wouldn't
consider excusing yourself to vomit (and smelling of puke
thereafter) attractive at all.