three months is a long time to veg, the plant will double to triple in size when it goes into flower, dont worry about watering a good bit in one go, the problems start when you water too soon after a good watering. start to judge for your self how heavy the pot feels after a good watering and then dont water until it feels light.
dont worry about the mg soil, a lot of people use it and have some good results if you use it right. you could do with getting a t5 or a t8 down either side and then use the 26w cfls up the top of the plant. when that plant goes to 12/12 its gonna get big!
lu