My 1st Ever Grow - Purple Kush and Ak47 (both auto balcony grows)

Hi rollitup growers, just looking for some advice / opinions on time frames and how my seedlings are doing so far.

My seedlings are both planted in 'BioBizz light mix soil' and are about 16 days since germination.
1 is a purple kush the other Ak47 (got them mixed up when germination so not sure which is which, will become apparent when I start seeing some delectable purple buds from my kush though!)

Both are being grown inside next to balcony window in the UK, supplmented by some cfl light although not very intensive, they are probably getting about 5 hours direct sunlight (its been decent weather here lol i know wtf) and about 5 hours cfl light a day, so not as much as i would like but this will change soon. I bought a big 125W cfl light which I will be using soon.

Question 1: One seedling is slightly more stunted and has more yellowy leaves than the other looks healthy vibrant green albeit slightly stretched. What could be the reason for this seeing as they are in identical situations?

2: I have recently bought a large CFL 125W light with both light spectrums with built on reflector: should I ditch the balcony window position and just have the cfl on them for like 20 hours a day? or should the opprtunity for direct sunlight not be shunned? Its just it might be a problem having the CFL on the balcony window area because that my main livng area and I don't think the gf will be too happy to have her evenings basking in eerie cfl light, may kill the mood a bit lol. So I think its either direct sun but less hours , or more hours in another room with CFL light. What do you think?

3: When should I start fertilizing? I bought BioBizz grow and Biobizz flower fertilizers.

4: I feel i may be over watering a bit but not sure what is optimal. They are in quite large pots and read that seedlings need to be in slighty moist soil most of the time. Should I lean towards underwatering at this stage?

5: yield estimates, and any experience or insight into these strains would be appreciated

here are some pics of them 16 days old :

Ok so you cant really see from these pictures, but one plant is much more yellow from the center outwards(not the edges/tips) than the other and is less developed both in height and leaf development. Could this be a strain straight or is it a deficiency or mutation?

Thanks

Oh yeah and I don't have a fan on them but they are in a large well vetilated room, however sometimes I feel they may be over heating in the direct sunlight through the window with little air movement but I could just be being over prtective.

Also I haven't taken any PH readings. Is this a must.I know my soil PH from the infol on the pack (6.3) but I m just watering them with filtered tap water, about once every 2/3 days.

the fan is more for movement simulates wind which makes the stalks stronger and i usually water when the top looks dry auto's dont like nutes young and dont like sitting in water makes growth almost stop

the center yellowing is more then likely just new growth my leaves twist and bend until they fill out no worries
and in my experience most soils are a little to hot for auto's i usually cut them down with perlite indoors or sarghpum moss if outside

ill run through my process if i dont cover a question let me know
1 lite soil for seedlings i just use soil from a previous grow when i setup the pot i just dig a hole in the middle and fill it with soil that i harvested from <~~~~ been flushed for 2 weeks almost no nutes
2 auto's like drainage i use high perlite 50/50
3 lights are on a 24/7
4 im only running 400 w mixed spec
5 i dont start nutes until preflower and only use 1/4 strength if no change i increase a bit and use a feed feed cycle dont water as much just more once a day shot half or so of normal waterings
6 i switch to bloom nutes when the fuzz balls start if it needs to be small start bloom nutes at preflower

ph is not required but helps you control the situation
filtered tap water is good but you need to let it sit over night to release the chlorine