First of all, your friend has just one baby. That alone makes a WORLD of difference in the parenting experience. She has no way of understanding that, even if you try to explain it. Her perspective is of one little singleton girl. Secondly, there is just no way to compare kids. There are too many variables. My first child, a girl, hardly cried for the first two years of her life. I'm serious -- totally adaptable, chilled out, quiet kid. She turned into a willful, argumentative, controlling little darling almost the minute she turned 2. Her brother, on the other hand (he was born just before my daughter turned 2), came into the world screaming like a banshee. I hardly got a minute's peace for 3 months. He evened out into a pretty laid back kid and now, at 2, is a piece of cake compared to what his sister put us through.
Anyway, for the easy question -- the word now is to wait until 6 months to start baby food. You're on the right track! And, with twins, they were likely born early, so you want to be especially careful. On the other hand, if they are drinking tons of formula and act hungry a lot, 5 months isn't necessarily too early. Since you would only introduce one new food a week, they'll only be trying a couple of things by the time they're 6 months, so it really doesn't make that much difference. In case you're wondering what to start with, it's really up to you. I've heard to start with veggies first, then fruits. Mine both started with sweet potatoes. Just see what looks good in the little "1st foods" jars. Each kid should eat around a couple of tablespoons (maybe half of one of those little jars) to start with.
As far as them getting sick all the time, that's totally normal. Really! And again, there's no way to compare two kids, even if they're in the same family! My kids have been opposites in this too. I will say that if they have allergies, they will tend to get sick more often. The good news is that they outgrow a lot of that after they're 2 or 3 years old. My daughter has a milk allergy that caused a lot of congestion and that is totally gone now that she's on soy milk. My son has environmental allergies that no one has been able to pinpoint, so we give him Zyrtec regularly. I hate the idea of "maintenance medication" but it sure beats all that illness (he was on 7 antibiotics between Sept and Feb)! We are using some natural stuff as well and the combination seems to help (haven't had a problem in 2 months!).
Anyway, all that to say that NO ONE has it all figured out, no matter how good it looks on the outside. You are doing a great job meeting their needs and you shouldn't pay a bit of attention to what your co-worker does with her kid. When she tells you about her baby, you should just smile and say, "that sounds nice."