Yes, Mono Sebae are fine in Freshwater after they are a couple weeks old and most Sebae are purchased at a few months plus....They HAVE Bred in Full Freshwater
I would just wait until the sebae is large enough that the arrow won't look at it as food. Luckily the sebae will be very tall, so it will appear too large to fit in the arrow's mouth
I have never had a sebae nip at any of my aro's fins. Just keep them well fed and they tend to leave everyone else alone. The do love to chase one another though so I wouldn't recommend putting 2 together, 4 or more would be ideal.