Can a guppy and a tetra mate?

I do not believe so. I have never heard of or seen this occurring naturally.

No, this is not possible, guppies can only mate with other guppies.

Actually, yes. The offspring are all sterile males. So you can't get any further fish from the mating.

Do not be missled. Guppies and Mollies are totally different species and can not breed together. To say that a cross between a Guppy and a Molly is possible is like saying that a Human can be crossed with a baboon. It CAN NOT BE DONE naturally.

Melchisedec and Christino (the last contributor) are spot on. As with many living things,and in many of the sciences the 'taxonomists' are continuously re-defining and re-naming many genera and species. I know of some trees that have had their species changed 4 or 5 times over the last 10 years or so. I am quite sure that the new species name for the old 'Lebistes reticulatus' ie Guppy which is now 'Poecilia reticulata' may well be changed again and the old name for molly which is now 'Poecilia' was 'Molenisia sp. may also be changed again. It is quite obvious from simple observation that the two fish are not of the same species. Some taxonomists obviously need to get away from the microscope and use their eyes (observe) and use common sense occasionally.