You can trade with Envi FX with just $5, it all depends on the leverage you choose.
For example, if your leverage was 1:500, you could buy 0.01 lots of GBPUSD at 1.18956 and the trade would cost you $2.38. Whereas if your leverage was 1:100 it would cost you $11.90.
You would be limited in the pairs/instruments that you could trade with as some pairs/instruments have higher contract sizes or lower leverage.
Indices and commodities have lower leverage so you would need more money to open a position.