This depends on what type of guppie.
and...
As long as your guppies and goldfish are relatively the same size, you should be ok. Goldfish seem to be more "docile" than other species. Keep in mind, goldfish can get big depending on your tank size and the number of fish they tank with. I've have had guppies with mollies, goldfish, and platies before and never had a problem. I would keep an eye on everyone though, just to make sure you don't have any tail-nippers. - Darkhorse
I have an Oscar Cichlid in the same tank as mine and I also used to have it with guppy's.
A male guppy can be in the same tank with its own kind and/or with almost all peaceful fish smaller than 5 inchs.
no you can not put a female and male fighting fish in the same tank because they will just kill each other
Yes, it is possible for a guppy's tail to be bitten off by another fish in the tank.
They can be in a community of other tropical fish of the same size.
no because they will end up killing each other
Fighting fish are territorial and can be very aggressive, they will kill other fish if you put them in the same tank.
The Gravid female will have the dark "gravid spot" in front of her vent. I can however assure you that if there has been a male in the same tank with a female guppy for any reasonable length of time she will be gravid.
Yes, do not go get another guppy unless it is the same gender they reproduce really fast. But fish are better with another fish.
No, because they are fighting fish and one of them will kill the other.
2 or 3
Yes, you can just not have 2 male bettas together.