Unique Constraint with foreign Key

Hi all i am using postgres and torque
I have a column x in Table A of type varchar and NOT NULL. I want to
impose a unique key constraint on that table and column y is the primary
key.
I have another Table B with column x. can i make this column x as a
foreign key on column x in table A.
or Can i have varchar types  of size 50 as primary keys in Postgres.

please let me know

--Mohan