I only have two children, so have only tandem nursed once. I can tell you my experiences and about what I have read, but much like breastfeeding in general is different for each mother-baby pair, tandem nursing is, too!
First, you definately don't need to worry about colustrum being used up. Just like women who leak during pregnancy don't need to worry about not having enough! Your body produces colustrum until it recieves the biochemical signal (from a variety of sources, mostly from the placenta detaching from what I understand) to start producing mature milk. That mature milk takes different amounts of time for different women to come in, but your baby will definately have colustrum.
Since women can (and do) nurse multiples, you also don't need to be worried about baby not getting enough milk. In fact, for most tandeming moms they are worried *more* about their older child's sudden weight gain! With DD (2.5 when DS was born) she gained maybe 2 pounds during my pregnancy and then gained 5 pounds in the few months after DS was born. There was just suddently so much good, fatty milk, lol. Because they will both be telling your body to produce milk for them, there should be enough for both (especially if you've never had supply issues). From what I have read, the milk will be aimed at your infant (fat/sugar/protein ratios) but it is still wonderful for your toddler!
Lack of engorgement is also a real "plus" for many tandem nursing mothers. Because your toddler is probably more efficient than a newborn, they both bring your milk in faster *and* can empty your breasts better. You do need to watch out for your own health, though--- producing milk for two can take a lot of energy and calories!
Night nursing was hard for me, too. When DS was a bit older, I started allowing DD one really, really long nursing session in the evenings. I don't mean only, I mean that that one was not up for DS interrupting, kwim. If there was a real emergency we could cut it short, but otherwise she knew that at the end of the day she could nurse and nurse and nurse if that was what she needed. She still nursed a lot during the day (and night), but it was easier for both of us to cut those sessions short, or postpone them because she knew that she WOULD get to nurse to her hearts content eventually. For me, nursing her in the morning was generally just easier than any other option. It meant that I didn't need to jump right out of bed and get her something to eat while DS was wanting to snuggle.