It is hard to advise you about the ingredients for your watermelon wine (especially in such a large quantity). I would first try and gauge how sweet the juice is (watermelons vary a lot).

If you raised the watermelons - did you have to water them a lot, were they very large or just a large crop - small, concentrated watermelon produces the sweetest juice.

Sieve the juice before you start and before you taste it. If you are intent on using all juice and it is sweet to taste, start about a gallon off with some yeast and the juice from 2 lemons. If it is fermenting well a week later (steady bubbling that hasn't subsided since day 2), start the rest.

If fermentation stops abruptly during the week, add about 1 cup of sugar and the lemon juice to another gallon. If you have to add more than a cup and a half to keep it fermenting a week it probably won't make good wine.

If you do a good job sieving it at the beginning, it should be crystal clear and no further chemicals need to be added (and quite potent !) after it stops fermenting. Make sure ALL action has stopped before you decant it into bottles.