You will usually notice a grey, torn look at the tip of the fin (it will spread as the fin rot progresses).
yes the guppies might eat her fin because they it looks like food .
No,but guppies(livebearer types) can live with small sharks like dwarf shark e.g.
because people think it will make them look powerful and fancy
well, for guppies, if the small fin under the tail fin is long and ribbony, its male. if its wide like a tetra's, its female. ( Its the same for bettas.)
On a guppies underside they either have a triangle shaped fin or a fin that looks long and pointy. Long and pointy is male, triangle is female. http://thesmileyfish.com/wp-content/uploads/2008/03/tropicalfish010.jpg
1 inch of fish per gallon so you about 18 but you will have the guppies eaten or the goldfish's fin bitten off as goldfish like to eat fish that are smaller than them (other than goldfish) and guppies are fin nippers so I would be careful or not keep them together good luck
It is a misconception that betta fish do not get along with other fish. While intolerant to their own species, bettas can get along with other fish so long as they are not easy to mistake for another male betta - such as fancy guppies - or fin-nipping fish like many tetras that often go for long-finned fish. Female bettas can be housed with guppies, however.
This is with every fish, the males bottom fin is like a spike as the females are like a circle.
Male guppies have a pointier anal fin and a bigger tail than the female and the female is larger than the male.
You can tell them apart by the gonopodium, a tube-like structure at the front of the anal fin. It is only present in the male of livebearers. Common livebearers include mollies, platys, swordtails, and guppies. the females have a fan like fin
the female orca's dorsal fin is curved and the males dorsal fin is straight
From your description you have a male and a female. The only way to sex livebearers is to look at their anal fin (the fin in front of the anus). In the females it is a triangular fin. In the males it is narrow and long and comes very much to a point. This is called a "gonopodium" and is the male livebearing fishes equivalent of a penis.