Abdominals may be done either first or last. If they are done first, they can be considered to be part of your general warm-up, which should precede every workout session. Some prefer to do them last because they are major support muscles for every exercise we do. I do them last, but only because that is what I want. It is a fairly arbitrary decision to be made by most of us.
Some people do a legs-only workout.
Again, it's your choice. I do mine first because I don't like legwork, and want to get it over. An important aspect of one's training program is recovery time. Progress from weight training comes during recovery time, not during the actual training time, therefore, it is crucial to the success of any exercise program that adequate rest is factored in. I'm going to work twice as hard and really make some progress.