Hold on, maybe I am a bit outdated (or confused) here - have they expanded the frigate and cruiser count limit then? Because my mod strikes for balance with vanilla races I haven't actually tried to add more types of ships than there are per class - I just seem to remember that you couldn't add any new ships, frigates or otherwise, but the last time I personally tried that was way back when modding was just starting off...
So if 7DS can do it, Arkleseizure, disregard my comment.
However, if the unit simply does not meet the research criteria, the icon should show up, just be greyed out. There is an easy way to test that - go to the ship's entity file and remove any building prerequisites. Also make sure you have increased the listing count in the player entity file when you added the extra ship. If you forget to do that the game may crash or it may simply ignore any extra entries beyond the number specified, so that may also be the reason your icon is not showing up.
