Grow lights- I move them out and into the greenhouse at the end of January usually. This is partly due to temperatures but also light levels, which improve a lot over the month. I harden them off and plant them out in May as a general rule, but you need proper spring like conditions and a cloche or fleece on standby for late frosts.
Feed - yes. I use a weak Baby Bio solution when they are in the house, because it lives under the sink and I use it for houseplants. They get weak veg feed once out in the greenhouse.
Potting on I leave until they straighten up and start on a second or third leaf. If you feed them, they can stay in a starter seed tray or pot for a while without harm and they are less fiddly to handle