I work on my abs twice a week, usually I do it on my leg, chest days.
Can I train chest and abs in turn?
Example:
30min chest
5min abs
10min chest
5min abs
5min shoulder
5min abs
5min shoulder
5min abs
Or should I work abs after I finish chest?
If I do them in turn, they get rested so I have more energy to do more sets, but I don't know this will affect the result or not.
What is the optimal amount of sets for training?
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.
Let's try to break-down what your asking.
This is a highly debated topic in the gym (and in labs). Recently there have been a number of meta-analysis studies, notably Krieger JW's study, that reveal a superiority in effectiveness with 2-3 sets of training vs. a single set. The same study however shows that over 3 sets (4-6 sets) has no advantage over 2-3 sets.
That being said I have always been a fan of single set training. I have found it effective for myself for both strength and hypertrophy training. I have not conducted any sort of serious analysis on myself however. When I train others I always recommend multiple sets, if not just for them to get more comfortable in the movement. It's also important to note that single set training is meant to bring you close to failure (or to failure) and be perhaps a more intense set than the multiple sets would be. This was originally propsed by Arthur Jones in the 70's.
IMO - Find the amount of sets that you feel are most effective for yourself.
I noticed that you broke down sets into time. There are 2 amounts of time that should be separated.
Time under tension(TUT)- is the amount of time the muscle is under stress.
Rest (time)- the amount of time you rest between sets.
If your going to follow the multiple set method for hypertrophy, keep your TUT close to a minute and your Rest under 2 minutes.
Building muscle burns fat. I wrote about that here. Adding cardio to your weekly routine will further increase the rate of fat loss (providing you train with the right intensity for long enough).
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