Let me try to clarify few things to you so that you make a better decision at selecting subjects.
First, keep in mind that CSS is all about scores/marks. In the end you will loose allocation by even a single mark, or be allocated to a lower group by just few marks. The difference between people allocated to DMG/PSP/FSP and railways/postal/OMG is only about 30-40 marks. So, marks matter and so so does scoring potential of subjects.
Second, you are punjab domicile which means you will have the most fierce competition. Every year many punjab candidate finally qualify but donot get allocated( myself an example). So, agian scores and scoring potential of a subject are important.
The above arguments have been put up to tell you that yes you should try to go with the generally high scoring subjects.
POl. Science: Its a good subject and it helps you broaden your overall vision with regard to matters of state hood and provides you with a greater understading of the working stat system.
On the other hand its generally not a very scoring subject. A very good attampt would fetch you 130 marks, very few cross this limit. The subject is very vas and too subjective. So the scoring is difficult.
Indo-Pak History: Out of all the history subjects this seems the most easy but is the most tricky. Only opt if you have a very very good command over this subject. If you can cram extenxivkey and you feel you will be able to pen down alooot of material, and in a different way. It can actually create disaster as well a swhen it scores low it compleltey destructs.
Sociology: Its a good subject that will help you develop alot of concepts that you may use in other subjects and papers as well. It has its ups and downs, but usually it can fetch you between 55-65. Its a safe subject.
Journalism: Its the cheescake of all subjects of CSS. If you finally decide not to take pol science you should!!! take this subject. With your BBA background you will be able to do the questions of advertising, PR
, Communication very easily. Its is a must take. A good attempt can easily get you 60+ abd you can aim at 70 as well without much diffilculty.
Const. Law: a good subject that if taken with Pol science becomes even easier as you cover constitutions in pol scinece paper B. But again its not very hig scoring. It has the average of being between 50-55.
Psychology: a good subject that is very objective, not even lengthy and that can easily get you score of 130+. Good attampets can get you between 140-155. It does not have biology in it, only avout 2 chapters rlate to that out of some 16-17 for both the exams.
It is you yourself who has to make the final decision but with my own experience of CSS i would like to recommend some subjects to you and you may choose as you like.
Psychology( read the syllabus and see the pats papers)
Business Admn( with BBA background it will not be diffilicult for you, i revised fro 2-3 days and scored 61 in 2010 and 78 in 2011.
History( if you want to opt a history then i would suggest go for british/islamic history rtaher than indo pak)
Law( the best subject in law is intl law very high scoring and not difficult as well, then u may go for muslim law and then c law.
Pol science( i would suggest it would be better not to opt it as its very tricky with regard to scores.
JOurnalism: it should be part of your selection, short and easy to get marks.
Punjabi: If you can write in urdu/punjbai font then its a good good choice, just cram more and more verses to quote.
Psychology+ Socio+ Journalism+ BusAd+ Intl Law
Psych+Journalism+punjabi+ Bus Ad + IR
British/Islamic History cab be taken in place of psychology