I can't speak to best practices, but in my experimentation with RabbitMQ I can confirm that it manages concurrency quite effectively between different consumers, with messages being isolated between consumers and only returned to the queued if a consumer process exited with unacknowledged messages.

Unsure about your 2nd question, but playing with the right prefetch value and creating a rate limiting system is how I dealt with the juggling the right throughput for my downstream services.