I waited hoping someone with more experience would chime in, but they didn't so here's my opinion.
1) It depends. Before working your muscles, you should warm up. Push-ups are great for this, but so is doing a light set of whatever exercise you are going to do. I've used push-ups at the end of my chest workout, when I wanted to give it that little extra bump. I figured it was easier to turn my head when my muscles failed and I came crashing down than it was to rely on my workout partner, who was just a spent as I was, to hold the weight. Normally on those days I seem to make some great gains.
2) I'm not a personal trainer, but I normally do two or three different exercises for large muscle groups and one or two exercise for the smaller groups. I also work complementing muscles, like back and biceps or chest and triceps. I do the larger group first then the smaller group. That way the smaller muscle group is warmed up when I do the larger group. For chest/tris my workout would be: 4 sets of about 10 reps of dumbbell press (one set is warm up), 3 sets about 10 reps of dumbbell flyes 3 sets of about 10 reps French Press and if I feel energetic 3 sets of press downs. The exercises for smaller muscle group changes every time. The larger group changes every two or three weeks. I'm just maintaining my strength and size, so I'm not enthused about being in my workout room for more than about 40-50 minutes including cardio.
I'd suggest buying a subscription to Muscle and Fitness. They have articles about different exercises and how to properly do them. They also give the routines the pros use, that can be easily modified for your purposes.
But when a long train of abuses and usurpations, pursuing invariably the same object, evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new guards for their future security.
Translation for the intellectually challenged: If the government screws the people too much, it is the right and duty of the people to revolt and form a new government.