first of all congrats, nice to know other people are happy to have 6 kids as well!
we have 5 boys (oldest is 11, youngest almost 3) and baby girl 9 months old soon. they were all breast fed for various amounts of time, DD is still on the boob last thing at night, 1st morning feed and sometimes in the afternoon.
she had tongue-tie and problems with both bf and ff, and took 6 weeks to get back to birth weight. I had to spend a lot of time pinned to the sofa so I bravely asked people to help with the school runs, but despite all the help she had to give her formula from about 4 weeks old.
she's ok now, healthy weight (just started to crawl, so cute!)
DS3 and DS5 had tongue-tie as well - it does run in the family, so have this checked and rechecked if there are problems with bf!!!
it is daunting to think how you'll manage just bf (or indeed anything 
), so maybe mixed feeding from day one is the answer!
my 4th was jaundiced at 5 days, and I had to give him ff in hospital as he was too sleepy to breastfeed.
2 weeks later I had mastitis, then another a few weeks after, plus he was a C-section and I had 4 kids under five, school and playgroup runs 3 times a day, so I had to mix feed or I don't know how I would have survived!
I think you will need people to help you with things, so do ask for what you need and accept offers!
if you can afford it try and get some regular help - we have a cleaner who comes on Mondays for 2 hrs, best thing to spend £20 on!!!!
it's also worth to literally write down your daily/weekly routines, to see what's a must and can't be left out (like your DS's special bed time or homework) and delegate what you don't personally have to do or plain forget about everything that's not vital/important for the first 3 or 6 months.
I think my expectations are pretty minimal for now. I don't enjoy survival-mode, but it is what it is and I don't care what anyone thinks about my untidy house or if I forget things or cancel stuff or if I give the children cereal for dinner!(which they love...)
if your floor is not clean enough to eat from that's ok, that's what plates are for!! whoever eats from the floor anyway, stupidest thing to brag about, ever!
BTW I don't think I manage all their needs very well. Just today I had a major meltdown (screaming, crying, blaming everyone - just awful...), but they were giving me big hugs and kisses and told me I'm the best mum in the world, so at least they know I love them loads, despite whatever shit is going on! I guess that's all that really matters....