running the lights at night will prevent a nightly pH swing. Got to do with the alge not photosynthesizing at night and producing carbon dioxide. Running them at an alternatng time to your tank lights limits the effects of this.
Not a problem at all.
Give it about 12hrs of light a day and if possible run it while your tank lights are off. It will grow under lights with a lower kelvin rating (so around 6000k). Which T5 tube are you using?