I exclusively breastfed for the first three months of my little girl's life. I went back to work when she was 10 weeks old (full time), and pumped. I never ever pumped enough to keep up with her, so for two weeks after I started back to work I struggled and pumped extra on my days off to try to keep up. After that, I gave up on the exclusivity. I started mixing formula in with her breastmilk, at first just one ounce to 3 of breastmilk, to "stretch" it. As she got older and ate more, I could still only pump enough for about three ounces in a bottle, so the amount of formula went up. Now, at 8 months old, I just breastfeed at night before she goes to sleep, and she is formula fed the rest of the time. She never had any "poop" problems when I switched her over, and I think that's because it was just so gradual, or maybe because we're just lucky. I know some babies have a hard time switching back and forth between breast and bottle, but I don't think that's a majority. We gave her her first bottle when she was 4 weeks old and breastfeeding was very well established. Try to find a nipple that closely resembles yours, and I would even suggest trying out a bottle a little sooner, maybe between 2-3 weeks old (but no sooner). Mine also still likes slower-flow nipples-- we use Avent, and she likes the stage 2 nipples. I tried to give her faster ones and she about choked :) I think it's because she's used to the slower flow of breast milk. I'm pro-breastfeeding for as much and as long as you can do, but the bottom line is it's a personal choice, and a big commitment, and only you can decide if it's right for you and your baby. But I love it, it's way cheaper than formula-- and even a top of the line expensive breast pump still only runs about what one month of formula would run you. As far as types of formula, we started off with Enfamil Lipil, because it has the DHA/ARA hormones that are found in breastmilk naturally. We switched to a generic brand of the same type of formula for about half the cost, and no adverse affects to the baby :) It's all FDA regulated, and most generics have charts on the back that show they are equal to the leading brands. I would recommend that if you do start with one and switch to another, don't do it abruptly-- we did that once and her poop turned teal and became really hard! Just mix it gradually over a week, and your little one should be ok. To freeze breastmilk, I would recommend the Gerber brand zip-top baggies (although some people don't seem to like them). What I would do, and is pretty common, I think, is something like this: On Monday, what you pump you use for Tues, and so on. Any extra, you freeze. Before you return to work, start trying to stock up a bit extra, and also so you have some for that first day back. If you are breastfeeding every 3 hours, then 1 1/2 hours after you feed, pump for a few minutes. You won't get much out most likely at first, but if you consistantly do that every feeding every day, your body will resond and boost your supply. We never had a good stock in the freezer, but you need to at least have some to get through the first day or two back at work. I figured 4 ounce bottles for each feeding, because everything I read said "between 3-5 ounces each feeding". I would take a felt tip pen or marker and mark the date on the bag, as well as mark on there how much is in there (it's graduated on the side like a measuring cup, but I found it wasn't accurate), and then just toss it in the freezer! Breastmilk is good for up to 3 months in the freezer, but the composition of your fresh milk changes as your babies needs change, so I wouldn't try to stock up enough for 3 months, because your 6 month old would get milk meant for a 3 month old. It's only good for 24 hours once you defrost it though, and the hardest thing for me was dumping milk out that she hadn't used when I didn't have a whole lot to begin with. It's good fresh in the refrigerator for 7 days. When you defrost it, do it slowly, either in the fridge, or in a bowl of water that you change every few minutes. Don't microwave it, as it breaks down important proteins, and can cause "hot spots".
Ok, I know that's a really long post, and I hope I didn't overwhelm you. Please feel free to send me a personal message if you have any other questions. And remember-- breast is best, but formula never killed anyone! Good luck!