I have the same gap - my eldest is now 6 and the twins are 4.
How did the toddler cope - surprisingly well, no issues at all really. We did the 'House inside my mummy' book and bought her a doll 'from the babies' that she could care for herself, which she wasn't particularly fussed by - but she was just so proud to be a big sister! Making time for her wasn't too hard either - remember babies sleep a lot and they'll have you one on one then.
Sleep - I didn't co-sleep with any of mine, but we put the twins in together for as long as they fit in the same cot (not long!) and separate after that.
NICU - no experience of that one, sorry. If it's any reassurance, 50% of twins don't need any help after birth - including my smaller twin who was only 4.5lb when she arrived.
Later pregnancy - yes, I was bigger than with my singleton, but I actually found it easier - every pregnancy is different and a multiples pregnancy isn't necessarily tougher.
Breastfeeding - I managed to combi feed for a few months but frankly, I don't think exclusively breastfeeding would have been possible. And having a toddler is why I switched to full formula when I did. My toddler caught a horrible vomiting bug and wanted me to care for her - it was a case of either exposing the twins to the germs or letting DH take over feeding - an easy choice.
General coping - tip 1, take all the help you can get. There's a charity called Home Start that pairs up volunteers with parents in tricky situations; multiple births being one of them. I had a 2 hour per week visit from a volunteer who'd had twins herself (grown up by that point) and she was there to be an extra pair of hands with literally anything - cleaning, caring for the kids whilst I took a bath, an extra pair of hands to get them out of the house.
Tip 2, get them into a routine. With a singleton, I found it easier to go with the flow/whatever the baby needed - but with twins, having them feeding at the same time and sleeping at the same time makes such a difference.