As emphasized in Chuck's answer, this is quite a complex question.
I will try to answer your two questions, but not to do a review of your plan in the example.
"Or should I work abs after I finish chest?"
Usually you do core/abs exercises at the very end of your workout.
The common reason is to avoid getting too tired core muscles to keep a proper tension in your core before you have finished exercises that are more demanding.
When doing chest presses you mainly have to keep a tension in your back/shoulders, and it seems to be reasonable to mix chest press with abs.
"What is the optimal amount of sets for training?" is a very broad question.
It depends on what you are training, why you are training, your current level, your ambitions, the amount of time for the exercise etc.
Generally spoken, it is the number of repetitions, which count. If you can finish all repetitions unbroken, then of course you should prefer to do this for increasing your endurance, saving time etc.
However, if you can do all repetitions unbroken, then wasn't the load too small?
Many factors limits the number of repetitions you can do without a break: Your VO2max, configuration of muscle fibers, ability to keep core tension, level of muscle restitution since last workout, the will to push your self including ignoring muscle pain.
These factors may vary from workout to workout, and that is why people have personal trainers, training partners etc.
Staying on your own, you will have to push yourself like telling yourself "If I can do all planned repetitions in one set, then 10% less reps is o.k.", and decide the partitioning of the repetitions while doing the workout.
Finally, I will comment that it is very wise of you to question how you mix your exercises.
In many workout styles, e.g. crossfit, you intentionally mix the exercises and never redo a specific workout unless for performance testing.
Perhaps it is time for you to try something new. What about parkour, crossfit or calisthenics? They will have workout plans ready for you.