It seems you've mostly answered your own question. The only protection a firewall can give, on an isolated network, is from computers/users of the same network. Your firewall won't protect you from Internet hackers - the isolated nature of your network already does that.
What it will protect you from is malicious users or software installed on other systems within your isolated network. As you say, this can help prevent a user on one system from connecting to other systems via SSH. It will also help protect you in the case that one of your systems gets infected with a worm which tries to automatically propagate across the network.
For the highest security, you want your systems and users to be restricted so that they are only capable of performing actions which are absolutely necessary to fulfilling their business function. Firewalls are an essential tool in implementing these restrictions, even on isolated networks. However, it is up to you to decide if the risk (e.g.: of malicious users on the network, or another system being infected with malware) is really high enough to warrant the added complexity and administrative overhead of using that tool.