Tomatoes are happy to grow at about 15C, once they have germinated. If they are too hot they will get leggy as well as damp off. Once mine have their first true leaves I pot them up and they live on a sunny windowsill during the day, then I bring them into the room and they sit under my reading light for the evening. When on the windowsill, I put foil behind them to reflect any sunshine onto the backs of the plants, to maximise how much light they get. I think that too cold a temp when they are small will affect the crop later.