Unless your Hardware firewall specifically says so it only protects against inbound attack.
Any malware that manages to get past your defences will have free reign to connect to the internet to either download more of the same, pass your personal data (sensitive or otherwise, user names, passwords, keylogger retrieved data, etc.) or open a backdoor to your computer, so I feel outbound protection is essential.
yes, it´s enough for a small network (10 computers) but if you want security, install a firewall and antivirus on all the computers connected to the network