I think, it is allowed. There is no restriction that you have to take PCM only but you realise that there is no KCET for MBBS and BDS courses. Its through NEET. There is KCET for some allied courses. So while filling the application form, you should have taken Biology. However, the dates for this year are done. So if it is for next year, all the best! If not, you can't take PCB if you have not filled the form accordingly