Help in my project

[John]

1- The terrarium is large enough to house a group of E. tricolor or D. leucomelas?

You could house 3 or 4 leucomelas in there. I've never kept Epipedobates so I wouldn't presume to advise you on them.

2- What lamps should i use? and what is the potency it must have?

This is really personal preference. For terrarium plants you will need something in the daylight range (5000-10000K - I prefer 6700K). I like fluorescent bulbs, preferably T5, then perhaps T8. Alternatively, you can use CFL bulbs.

3- What heater should i use? i want to avoid to have something buried because if it gets damaged it will be a pain in the *** to repair it, a heating mat in the side of the terrarium will do the work? and what is the potency it must have?

It depends on how much you have to heat it. If you have a false bottom, you can submerge an aquarium heater and use that to heat the whole terrarium. Alternatively, heat cables (like those sold for plants) are ideal. There are several variants - Hydor make the most well known (and most highly praised, to my knowledge).

4- Which is better for a beginner D. leucomelas or E. tricolor?

I would say leucomelas.