The limitations are most likely on price. For the 200B messages they've already processed in the last 3 months, that would be $100,000 total on just the SQS FIFO queue, or $33,333 per month. And that's not counting data transfer.
As long as everything is in ec2 data transfer will be free. You're cost calculations are also off base. You'll need to send, receive and delete every message that you process via SQS. These can all be done in batches of 10. So it's 200B * 3/10 * .50 / million, which comes out to 60k over 3 months. Still not cheap, Kinesis is probably the better option in this case if you want an AWS managed service.
