Pumping Questions??

So, I'm almost 4 weeks pp, and even though I don't go back to work until 10 weeks, I want to start pumping and build up a supply.  It's quite the commitment, and I have a couple of questions.
1.  Was your pumping results less than mediocre at first?  I know I'm going to get less than a normal pumping session since I'm pumping about 30 min after breastfeeding (little one still likes to eat every 2 hrs), but I still feel like I'm maybe getting a 1/2 oz a session.  I've read thT 1.5-2 oz is normal when still breastfeeding.
2.  I know you can mix cold with cold when storing, so if I'm not pumping a lot right now, is it okay to mix from several sessions??
3.  Little girl pees all the time, so I know she's hydrated, but these pumping results make me worry that my supply isn't there :/ like it should be