Okay, looks like I found answers to my questions! From what I've found on this site, nutes can be given to the girls every third watering. I watered them last on Sunday, which means the soil should be dry around Thurdsday; plain water again.

Next watering after that should be around the following Sunday, so I'll give the girls another shot of diluted molasses then. 3.7 litres per gallon, my watering can holds 4 litres filled to the top, so my concerns about using too much molasses the last time were unfounded.

So...awaiting the delivery of the SMT pure red LED light...supplemental red spectrum and no extra heat won't go amiss. Agreed the HPS should be more than enough, but according to what I've been reading in the Grow Logs, there's no such thing as too much light!