Wow! Congratulations on the beautiful family. I have a 28 month old and 6 1/2 month old twins. I obviously didn't have as many at home when the twins arrived and my oldest wasn't quite as little as your youngest, but can tell you what I experienced. First, I'd be realistic with expectations on breast feeding. I fed my first, with no problems, until 1 (only stopped so we could start trying to have our second- and needed a hormone shot, which I couldn't have while nursing)... I was bound and determined to nurse, which I did 50/50 until 4 months, when I ended up letting it go. I'm not saying it can't be done, but you need to be very organized- one thing you may have going for you is the older kids who can help with your 8 month old. My oldest was 21 months when they were born (he is a climbed and VERY active)... so having to give him attention as well, I simply found it hard to physically eat and drink enough to keep up with the demands of breastfeeding two... I'd forget my water next to the rocking chair, etc and realize hours later I hadn't been drinking- was just so busy with the three. I ended up in the ER one night with extreme nausea and vertigo (dizziness)- turned out to be dehydration. I was bound and determined to increase my water/ food intake and get to 100% breastfeeding, but with pumping extra (recommended with preemies)- feedings, etc. it was too stressful. I hate to this day that I couldn't make it work, but the upside is the amount of flexibility I have now is a lifesaver. I know it would have been different if they were my first, but it was what it was. Whether you plan to nurse or use formula, I'd suggest filling out the card for multiple births you receive at one of your OB visits. I received a case of formula from both the makers of Enfamil and Similac... which is something. (They should contact you roughly 4 weeks after anticipated due date to confirm what type of formula they're on and it's shipped right to your home). I'd do it either way- it's has a long shelf life, and if you end up not using it, you'll find someone who will put it to use. (One last thing on this- if you do formula, the best place to buy is Sam's. A 38 oz can of Enfamil Lipil is $31.88, vs. a 25 oz can for $25.88 (average price) at Target or other stores. Similac is priced similarly.
The second piece of advice I'll offer is, when the babies are older, to learn to put the down drowsy. I was so enamored with my first (and off work until he was 7 1/2 months old) that I held him all the time- and with him being nursed, he often fell asleep while nursing/rocking. We eventually had to do a modified "cry it out" which was awful. I read in "Healthy Sleep Habits, Happy Child"- as they develop sleep patterns, to put them down drowsy- which has made a huge difference.
Finally, use your oldest kids to help in any way possible, as others have mentioned. My oldest has adapted beautifully to the twins, but find that the more I have him help the better he is. He loves to be the big brother and Mommy's helper...
Good luck!