online schema change is absolutely riddled with crippling bugs if you use foreign keys. Just a heads up, we've had to basically take everything it does and make an in-house version. The idea and execution are great, when it actually works.
Foreign keys are a major problem in MySQL if you do lots of writes, especially bulk writes. As is auto-increment (table-lock for load data infile, yay!).
We allocate our own IDs, and disable foreign keys for bulk loading.
