Dear Yong Peng,

y> manussa -- man, human being
y> nara/purisa - man, person

y> I have also looked up the PTS dictionary but still have some doubts.

It can be helpful to look up such words in Monier-Williams Sanskrit
dictionary
http://www.uni-koeln.de/phil-fak/indologie/tamil/mwd_search.html
http://members.ams.chello.nl/l.bontes/sans_n.htm

y> 1. Do we use purisa for the masculine gender?

Yes.

y> 2. Is there a preference, in the Tipitaka, for
y> man/person (singular) -- naro over puriso/manusso

naro - 606 times.
puriso - 3469 times.
manusso - 317 times.

y> men/people (plural) -- manussaa over naraa/purisaa

manussaa - 2622 times.
naraa - 241 times.
purisaa - 568 times.

Metta,
Dimitry