B.Com

Yes, after completing your graduation, you can do MBA. For that, you need to give MBA entrance exam such as CAT, XAT, CMAT, NMAT, MAT, CET, and then you will get admission in MBA College. And, you can do PGDM in finance field and you can also do Diploma or certification course in finance field.

Hi,
Yes, Maths is taken as a compulsory subject for B.Com (Hons.)

Every collage has it’s own entrance test, which will help you to get some benefits, if you pass those test.
Best collages for B.Com is:
Delhi Universities Campuses
Amity University
Lovely Professional University Etc.
I should suggest you should do:B.Com honors.

No, Mathematics subject is not compulsory in class 12th for B.Com Admissions.

The dates are not official yet, but you can apply through this link http://erp.loyolacollege.edu/loyolaonline/application/loginManager/youLogin.jsp

Hi,
Good colleges for B.Com include:
St. Xavier's, Mumbai
NM College, Mumbai
Mithibai, Mumbai
KJ Somaiya, Mumbai
HR College, Mumbai
Ra Podar Mumbai
Jai Hind
DG ruparel
Elphinstone College
St. Andrews college.

Yes, you can take admission in St. Xavier's College, but subject to your performance in board. And, may I know the reason why you want to change your field of study it may backfire you.

It can be possible if you change the college within the same university. Its quite difficult to change the college, you will have to give a reason for that.
All the best.