Also short on experience here but nothing jumps out...

However, would highly recommend seeking a developmental psychologist with experience in giftedness to bounce things off of and get assessment if needed.

The playing side-by-side is fine I think until 4-5 years old. It may be too early to tell regarding that one aspect. There would have to be other signs.

Be able to read early would be one of the signed of giftedness. However, some gifted children don't necessarily read early. But once they do, they learn at a fast pace.

There are times when an older child will appear to be/play like a baby for fun or to be like his little brother etc. That happens often, even much older kids.

Just some initial thoughts though definitely not much experience in this. But like the other posters have said, good for you for researching more about this.