Well, we've done everything they say you shouldn't do. We adopted out of birth order, and our adopted child is also similar in age to one of our bio kids.
DS was a few days from his 5th birthday when he joined our family. At the time, our bio kids were 5, 3.5 and 2. Although it did take a few months for the kids to adjust, it has turned out pretty well for us. Honestly, it took DS a lot longer to bond with me and DH than it did for him to bond with his siblings. I think that having siblings similar in age to DS helped him to feel more accepted and comfortable with our family than he would have if he didn't have siblings to play with. He was accepted by his siblings almost immediately, and learned a tremendous amount from them.
At first, I did have to be careful and make sure that my oldest DS (bio child) had enough time away from his new brother, because DS2 (adopted) would want to be with his older brother ALL THE TIME. Scheduling playdates once a week and allowing DS1 to play with his friends alone from time to time really helped DS1 to feel that he wasn't being smothered by his new brother.
As for competition, etc- it really hasn't been an issue. Although my sons are 7 months apart in age, they are at very different levels when it comes to maturity and academic level. They have what I believe to be a typical 'big brother- little brother' relationship. DS2 (adopted) and DD1 (bio, 14 months younger than DS2) are at similar levels regarding academics and maturity. But because DS2 and DD1 are different genders, and have different outside interests and different friends, it hasn't been a problem thus far.
I will be honest and say that we DID have a really difficult transition when DS joined our family. I don't attribute much of our difficulties to the fact that we adopted out of birth order. Our difficulties had much more to do with DS's age when he joined our family, his resistance to attaching to us as parents, and our inability to communicate effectively with him to start with (he didn't speak any English when we adopted him).