Hey,
Yes,you are eligible for admission in the Bachelor of Commerce even if you have completed your 12th from Science stream. The basic requirement is that you must have studied any of the following:accountancy,book keeping,business economics including business Business Mathematics,business organization,business studies,commerce,cost accountancy and principle of management,economics,financial accounting,mathematics,office practice.
Most of the colleges require minimum 50 percent or 60 percent in 12th.
Hope this helps.
Good luck.

Yes you can easily start your graduation i.e. bcom.after your 12th in science.There is no problem in it.Many students shift to Arts stream after 12 th in commerce or science.That's all depends on your interest and future goals.So don't panic.But I would suggest you to do bsc. as it is just the revision of your 11 the and 12 the concept in detail.But you have to face problem in bcom.while studying.It is just a personal suggestion,Just work according to your will.
Thanks.