A 25watt heater should be fine in a plastic tank, although it will be absoultely essential for it to be 'food grade' plastic or a proper purpose made plastic aquarium. Any other type will either melt or over time release very tiny amounts of nasties that will ultimately poison any fish.
The answer to the second question about suitable tankmates for a 20L betta tank is a bit of a contentious issue to some people. An awful lot of people would say that in a 5 gallon like that to leave him alone. Others will say various types of small fish are okay, others will disagree etc. It really depends on what you are looking for. A couple of Pygmy Cories, or Panda Cories, would do okay, an apple snail or 2 would also be cool, as would shrimp as long as they are big enough not to look like food to the betta whom may have a curious nibble at them.