livebearers & plants?

  I recently set up a 30 gal tank with the main purpose of propagating Java
fern and possibly some crypts.  This is a relatively low-tech tank with 2
 30W Vita-lites, an Aquaclear 200 and a Fluval 3 (for water movement). I haven't
added any fish yet.
     My husband has long wanted to have some livebearer's, probably guppies
and/or platys.  Will I have trouble creating conditions acceptable to both
the fish and the plants? How soft can the water be for livebearer's?  I have
read that mollies are good for cycling a plant tank--do you ever put salt in
for the fish--I wouldn't think the plants would like it much.  Thanks for any