Either the little hydor heaters which will sit inside the tank or you could use the reptile mats as they are water proof, place it under the tank and you can sit a few tanks on it if it is a very long mat.
Both will keep the ambient temp of the tank at a reasonable level.
Yes!
All they are is acrylic. Just the same as some tank kits.
It won't bother the plastic at all. It won't overheat.
In fact, you can use heaters to heat tupperware! Trust me, heaters can be used in practically anything.
You cxan use the 25w thermostat controlled heaters as well... I was in a bit of a jam a while ago and needed to heat a 1g container, only hetare I had was 25w, it worked a charm, that particular set up ran for a couple of months with no probs at all
10 l isnt the biggest size out there. Petsmart makes a 20 l critter keeper. Well, not so much petsmart as the company All Living Things. Petsmart just sells it.