I think you might be able to run it with those numbers, but it wouldn't leave much room for future growth. At 50-60 active users online, you're likely looking at less than 5 requests per second. This all depends on the resources available with the host, and how the provider goes about limiting them per client.
None of us can say with certainty if you would do okay on your particular shared hosting environment with those stats running flarum. My guess is it would be adequate, though not optimal (slow during peak traffic hours).