1. Is core switch and principal switch one and the same ?
ANS:- I have the following article that refers to core and edge swithes. Explains what a principal switch is also.
http://www.computerweekly.com/podcast/SAN-switching-How-to-configure-a-SAN-switch

2. As regards trunking a F port you can refer to the following:-
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/5_0/configuration/guides/int/nxos/cli_interfaces/trnk.html#wp1135692

3.What are the types of port rate modes and what does each type mean ?
As regards your third question refer to the following document. Its has a section expalining 'Port Rate Modes'.
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/5_2/configuration/guides/int/nx-os/cli_interfaces/gen2.html