Is it necessary that the house is airtight to get the best utilization?

Yes, it is! If the house is not airtight you will get ventilation that can not be recovered and you will not get maximum heat recovery. Moreover you do not avoid different irritants, coming through the windows and the indoor airclimate will not be improved. The trick is to make natural ventilation as small as possible. Else you will get too much ventilation.

By using ventilation systems from EcoVent, will you then need an extra heating element in the supply air?

If you have a system inside the climate shield it is not necessary, when the supply air is above 17-18 degrees. A supply air temperature of 17-18 degrees will not give any draught. If the ventilation system is placed in a cold loftroom, it can be necessary to have an extra heating element in the supply air, which might reach 16 degrees where the risk of draught is bigger.