i would think that 6 to 7 inches would be good for the first two weeks. once they get growing (now)they should really take off in the next few weeks. but your looking good.
i think you maybe be a little panicked about the time/grow rate, but soon they should be pushing towards your lights very fast, also keep in mind that they will grow some more when your go to flower them too

i wouldn't worry too much about the ph levels as your peat will bring it down. also a quick way to bring down your ph is to test and add some bottled water. Tesco's cheap water has a low ph and you can mix a little of that in with your tap water to balance it out. but keep in mind that your peat will bring it down again and you don't want to go too low