I bought a puffer thinking it was a green spotted puffer but its actually a figure of eight puffer. Can i keep it in a tank with 3 cichlids, 4 rosy barbs and 1 blue gourami?

i had one of these b4 ... and it never caused any problems with other fish ..... it did however have quite a nasty bite when it got big enough and drew blood ( from me ) on a few occasions

I would like to try to breed puffer fish. How do i tell the sex of a puffer fish? (figure of 8)

Figure of 8 puffers can be nippy as Dizzy found out .
Although they come from fresh water the do better with some salt added to the water.
As far as i know their is no visual way to sex them.

Could i lift up its tail to c if it has another little tail. Is there any way to sex them?

As Platty says, these have no apparent outward sexual differences, although the fish themselves know.
So far as I know the Figure-of-Eight Puffer is actually a brackish water fish which really does benefit from around a teaspoonful of Sea Salt (don't use table salt as this contains other chemicals to aid free-running in the salt cellar) to every gallon of tank water. Little or nothing was known about the breeding habits of these either the last time I looked into them, but that was pre-internet days - check them by entering their name in Google - so I would suspect getting them to breed might be a task of huge difficulty.
They are extremely personable fish though, if rather aggressive to smaller, slow moving, tank-mates. And extremely good at clearing unwanted snails!!! (Something I have loads of!!!).Perhaps I'll get a couple myself.
John

The figure of 8 puffer is 100% a brackish water species.

There is a lot of mixed reports on the web but the Aqualog book lists them as brackish.

They can even be kept in full marine conditions and should not be kept in freshwater as it will make them very prone to disease & significantly shorten their lifespan.

I kept them in my mangrove tank (see articles / breeding /breedeing four eyed fish)and they chased every smaller fish in the tank they even start to bite in the gonopodium of the four eyed fish.Eventualy I have to give them away.Greetings,Cees