Hi S.,
I know this is going to be long, I have a lot to say, and I hope it all helps. Feel free to message me with more questions. I will help as much as I can.
I have a 21 month old daughter and have a son on the way. We are planning on using cloth diapers for our son. We also are using the one size diapers. We bought them early and transitioned our daughter from disposables to the one size. We have mostly Happy Hineys, but also some Haute pockets. They both fit our daughter fairly well, and we have no problems with leaks, even with nights. When we first switched I purchased 1 of 4 different kinds of one size pockets (Haute, Happy hineys, BG 3.0, and a WAHM one) every diaper is shaped a little differently, so it is important to try different brands first, and see which ones have the best fit. They will also fir a toddler different then a baby, so right now these fit our 21 month daughter, and we are hoping they fit our son just as well. Every baby is different. Most leaks happen because the diaper does not fit correctly around the legs. Also any diaper that is lined with fleece (which most pockets and OS are) needs to be snug. You can put several inserts in a fleece diaper, and if the fleece is loose against the insert and not snug to the body they will leak. This is because fleece is not absorbent, but actually repels water. Fleece needs to have an absorbent fabric on the other side of it to help pull the liquid through. The reason diapers are made with fleece is so that the fleece in combo with a microfiber or hemp insert will pull the liquid through and then the baby doesn't feel wet because the fleece keeps sicking the water away. (Great for babies, bad for potty training) Both BG 3.0 and Happy Hineys have fleece liners.
As far as inserts go for your pockets, happy hineys come with 2 sizes of inserts a newborn size, and a regular size. BG 3.0s come with 1 adjustable size insert. All of these are a microfiber insert. Microfiber is very absorbent, but when you need to use multiple inserts such as at night, they can get bulky. For normal wear during the day, using the insert that comes with should be enough for an average wetter. During long naps or at night, you may need to add doublers. For nap I add the newborn insert to the regular insert in my Happy Hineys, and that is enough protection. As far as night time goes, I add two newborns on either side of a regular, and since doing this I have had no leaks. (although the best combination took several tries to get right) I also have a few hemp inserts. Hemp inserts are more absorbent for their size them microfiber, although they tend to hold less. 2 of my hemp inserts are less bulky then 1 microfiber and hold as much, if not a little more. I found a good website with a chart of the different brand and types of inserts and doublers sold, and their absorbencies. If i find it again I will send it to you. A infant or newborn 4-6-4 prefold diaper also works well for a doubler.
You may want to consider using prefold with covers for the first month or two. We are going to be doing this because we think our daughter will not be out of diapers yet, but also because no one size diaper has great review for fitting small babies, they say 8-35 lbs, but that means 10-32 or so. My SIL has twin girls who are just now 10 lbs, and some of her one sizes have started to fit them now, before their legs were to skinny, and would leak out of the leg holes. But again, this depends on your child.
I think I have said enough. I hope this helps. Good luck, and don't hesitate to ask questions. I have had experience with these diapers, and prefolds, and have spend countless hours doing research. I am happy to help.
After some searching here is the site I mentioned earlier
http://www.jilliansdrawers.com/store/choosinganinsert.html
By far the easiest way to compare most that I have seen. below are a few sites I cane across along the way, although a little more complicate I think they cover more inserts.
http://diaperware.com/information/absorbency.htm
http://www.babycottonbottoms.com/inserts_and_doublers.htm