Since the company does not have certified S.O., the Model S.O. would apply. I have tried to verify IE SO Bombay Act but could not download from net. But in the Model S.O. of Central Rules, the classification of employees speak only of Apprentices, there is no mention of company trainees. So, despite its absence, can a company still have Trainees is a moot question to be answered.
There are two types of trainees, one set of trainees are trained for further absorption in the company, more used for Management Trainees and the second is those engaged on year to year basis and after the training is over they leave, mostly technical trainees. In case you would like to adopt the latter course, have clear cut policy and training manual, appointment has to strictly follow the training schedule and the end date specifically mentioned and followed. It is seen that many organisations take trainees with the avowed purpose of imparting training, but they are used in regular production jobs, their appointment periods get extended, finally after lapse of sizeable period you have a problem in hand, neither they can be terminated nor you want to absorb them. Beware of this pitfall. To speak of my experience, in addition to Act Apprentices, we had certain Company Trainees taken every year and trained them as per curriculum and they were let off after training, but our Certified S.O. had a provision for it.