Capital gain exemptions

Q1 can we claim deduction of Sec 54b and 54f in case of residentail building used for the purpose of

a, commercial purpose

b, letting out

c, building used by the employees as rent free accomodation

Q2 If we have the option to take exemption as well as STCG/LTCG losses what should be claim first claimed exemption or setoff ?

Example

LTCG 100000

STCG (100000)

exemption u/s 54b available 50000