Don't add neons; they are tropical and will not survive the lower temps
that goldfish like.  A betta can thrive in cool water, but may not get
along with your goldfish; long fins may remind the betta of another
betta and cause the fighting response.  You could try 4 or 5 zebra or
leopard danios or some white cloud mountain minnows.  They are fine in
an unheated tank and will not fight with your goldfish.
Good Luck!
Scott B.
Carlos
Be careful.  We're headed into winter (in the U.S., anyway) so it's becoming less 
of a big deal, but I wouldn't keep neons in an unheated (more importantly, 
unregulated) aquarium during the summer, when temperatures may swing as much as 5 
or 6 degrees Fahrenheit in 24 hours.  Neons may do fine around 70--I'm not sure, 
I've never kept them at that temperature--but they're a little more susceptible 
to temperature swing-related white spot than many other species, in my 
experience.  
I missed the beginning of the thread, so I may be guilty of redundancy 
or other ignorance of the discussion here--but why don't you 
(danilo...@iol.it) invest in a heater?  
-- 
Bo Williams
wrwi...@ingr.com