Don't let the tap root get too long- just let it EMERGE but not get more than 1/4" before planting. Add some fine perlite to your soil too if you can; the lightness is important indoors.
T5 is much more intense light but I vegged for years, and still do in some parts of my grow, under t12 shop lights. Use the 'daylight' bulbs or 'cool white' or 'kitchen and bath' as a second choice. T12 is safer because if the plants grow up into the fixture they take longer to scorch, but t5 is brighter. t8 is a compromise and also popular.
You have to use a 'conversion bulb' rated for your ballast if you want to run MH in HPS ballast.
Temperature swings are not all that good for seedlings. 24/0 with stable temps is ideal (yes someone will disagree; many people swear by a dark period- this is personal preference). Swings lead to more stretch than youwould see without the night/day difference. Stability is GOOD.