Evening all.
Congratulations Poppet and well done BabyPoppet.
It must be very tiring waking in the night to pump and not even getting the cuddles
, you are doing so well! That first cuddle must have been wonderful. Have you been able to discuss Kangaroo Care with the staff and neonatologist? It sounds as if she is a bit more stable now and they may be able to allow you more skin-to-skin time with your DD which will not only help your milk supply but also help stabilise her heart rate and breathing and from what I understand, her weight gain.
In terms of tandemming your DS, your milk 'type' is determined by the hormones in your body post pregnancy and not by the age of the child nursing. IYKWIM? So you will carry on producing 'newborn milk' even if you do feed your DS. Supply should also not be a problem, in fact feeding your DS will probably help increase your supply as he will be more effective in removing milk from your breast than the pump (if he remembers how) and as supply is dependent on demand I can't imagine that it would be an issue if you were to feed him in between pumping sessions. However, you really need to do what you feel comfortable with.
The LLL shop has this book and this one about kangaroo care which might be useful.
Please keep us updated, we'd all love to know how you are doing.
Cherry welcome and congratulations. How far are you now? How old is your DC? The nipple pain can be really tough early on, but most of us have found that it subsides in the second trimester.
There are many myths about regarding bf and pregnancy and sadly most doctors are really poorly educated about it, even worse than MWs (and they are pretty bad too). It is true that bf releases oxytocin but there is no evidence to suggest that oxytocin release during pregnancy contributes to premature labour as it seems that the receptors need to be 'switched on' before the oxytocin has the effect of triggering labour on the uterus. There are some great articles here on Kellymom and Adventures in Tandem Nursing available from the LLL is fantastic too.
There is certainly no need to stop nursing at 24 weeks (or whatever arbitrary time given) unless there is a real indication to do so, so if you are happy to continue nursing, you don't need to feel forced to stop. HOWEVER, many mums find that nursing while pregnant is just too much for them and that they need to wean. Which is absolutely fine too.
Come and share with us, we love new blood 
PS: It doesn't sound like you are in the UK, am I right?