by "japanese fighting fish" i beleive you mean "seimese fighting fish" or "betta splendid"
in that case i would advise you not to put one in your tank
dispite what wrs says, betta splendids CAN and may very well be aggressive to fish of other species for dominance
betta splendids are very unpredictable fish
they can range from community to semi-agressive behavior depending on the specific fish
there is no way to tell for sure the true charactor of each individual fish, other then placing it in the exact enviroment (the tank in question) and watching closely for its reaction
in your case, you have more to worry about then just that
some of your tank inhabitants mentioned, should not be kept with such a species as a "betta splendid"
these are your guppies
being slow, and brightly coloured with large fins, it can almost be guaranteed they will be mistaken as bettas, and bullied until death occur's
the woman at the petstore was sort of right in the fact that you should not add too many fish too soon, but for different reasons
if you increase the amount of biological waste in your tank too quickly, it will result in a deadly nitrate spike
so to sum it all up,
i would say a betta splendid is not right for your tank,
although if you are very set on getting one,
they do not need a heater, light, airstone or filter
and if kept in a warmer room, fed live/frozen bloodworms and cleaned (water changed) weekly
they can be kept in a 2 gallon bowl/ jar
as for the bristlenose, i would not purchase one
they are not the same as cories and imo are not necessary in your tank
i WOULD get 2 more corydoras (cories) if i were you
they are schooling fish and should be kept in at least groups of 3
lastly i would not add any more fish to your tank other then the 2 cories, as mollies have semi-agressive potential, and require more personal space then other fish their size
leaving your tank as is, should help reduce any potential agression in your mollies
good luck :flower:
and if you have any more questions or concerns,
feel free to pm me