Well i don't think that there is any such thing called first come first serve basis anywhere, what i know that you will get admission in mhcet on the basis of your marks in exam. 

regarding the flost option, if there hapens such a cs that you don't get a good college in 2nd round, then i don't think that your flat option should be revoked. But you can confirm this by asking to the competent authorities of mhcet.