My answer is as under:-
(a) If you are a Diploma-holder in Engineering, you are not eligible for SSC CGL because Diploma in Engineering is not considered equivalent to Graduation.
(b) If you have a Bachelors degree, only then you can appear in SSC CGL.
(c) SSC CGL is a recruitment examination for filling up of various vacancies in Central Secretariat, Railway Board Secretariat, Indian Foreign Service Group B Non-Gazetted, Customs, Central Exercise, Audit Service, Military Audit, and similar other Group-B Non-Gazetted posts.
(d) You can't choose service on your own. It depends upon your final rank and number of vacancies in each service.
(e) It is a three-tier examination with elimination in each stage.
(f) So, nobody can be sure of permanent posting in Secretariats of Delhi. Only top qualified candidates can stay in Delhi.
(g) However, each of these services is better than State Government service where you are presently working.
Beware! I question everything and everybody.