I haven't read the books, but the stuff I've read about birth order seems to be pretty true. The first seems to be the bossy one, the baby is always the baby, etc. There are definitely personality traits associated with birth order that seem to be pretty consistent.
I agree musicpisces. I don't take it all seriously and let it rule things. However in regards to the fostering and adoption issue. I don't know if it would matter. They are who they are born to be. All of our traits are good/bad. The bossy one is a born leader. The baby is loving and affectionate. The middle is usually quiet but they are observant and know whats going on. So play up the positives of each child and gently help them with the negatives.