This is probably heresy, but I grew five sugar babies one year and five tiger babies in each of the past two years in a 25-gallon whiskey barrel. In each case, I got two melons per plant on three of them and one per plant on two of them (8 total). I would think you could grow two in 10 gallons. I would start with three or four and only cull all but two when the first fruit starts to form. There can be so many problems when they are young.

From what I read last year cucurbits don't like transplanting and do very well direct sow They do fine transplanted as long as they aren't left in a small pot more than a few weeks after they germinate. Once the roots crowd they won't do well. If you grow something and you aren't sure your warm season will be long enough to get a nice harvest of fully developed fruits, starting them indoors 4-5 weeks before going outside is an advantage. In some places commercial operations transplant cucurbits rather than direct seed.

Vines grow about six feet long, and each plant will set four to six melons. Plant four seeds in a square, about 3" from each other, and space each group about 8 feet apart. Thin to two strongest plants in each group. Sue

I had a feeling you were in Australia. :) When you say 25-30 degrees, I assume you mean Celsius, if so then that would be 77-80*F. Those temps are fine to plant watermelons outside. Just as long as the lows don't drop below 10*C. Soil temp also plays a factor but I assume the soil is sufficiently warmed. So plant them outside if you want. Just harden your seedlings off before planting. As for starting plants inside with lights, the length of time seems okay (keep it consistent!) but the light needs to be directly above the leaves of the plants. As close as possible without burning the leaves. A fan blowing gently on the seedlings also helps to make them stockier and less leggy. And if I were you, I'd do a search here about using peat pots for starting seeds. In theory they sound like a good idea (you plant the whole pot and don't have to disturb the plants' roots) but in practice they don't seem to be. Lots of people have had problems with them. The main issues are that they dry out too fast when starting seeds. They restrict the plants roots when planted whole in the garden because they don't compost quickly enough. And if the rim of the pots aren't completely buried below ground then the peat wicks away the moisture from the roots and dries them out. So cut or tear away the peat pot before planting. Rodney