Actually, the FS2 Open code has been updated to allow 3-primary and 4-secondary bank fighters to work for players. You can fire one primary bank at a time, or link all three banks at a considerable drop in rate of fire(worse than linking the usual two banks). No modifications to the POFs are needed, anymore.
If you're just trying to add them to an existing Terran/Vasudan-oriented campaign(rather than a Shivan-centric campaign, as there are a couple of those floating around; 'Tides of Darkness' being the only name I remember, ATM), then you will probably want to make duplicate entries, rather than edit/replace the original entries(I like to call mine 'SF Dragon#player' in the .tbm, for example; the '#player' part doesn't display, so it's indistinguishable in-game from the standard 'SF Dragon'), or otherwise override the special MediaVP "#Shivan" missile substitutions on the original Shivan fighters, as that will cause errors if the "#Shivan" missiles aren't flagged as player weapons. Duplicating the ship entries is less work, IMO.
If you choose the 3D ship models option in the FSOpen Launcher, the ship selection screen will have its own 'icons' generated from the actual ship model, and you won't need the ship interface art for the loadout screen(I think most mods nowadays don't bother with either, so they might be hard to come by). I just borrowed other retail primaries' icons/animations for the Shivan Light/Heavy/Mega Lasers.
Also, another tip; if you keep the ships' $Species as Shivan, and don't change it to Terran(to keep the red thruster glows), you won't be able to call in a support ship(though you can use ones called in by Terran or Vasudan wingmen), unless you also give the Shivans a support ship in the .tbm(you can just duplicate an entry to make a 'GTS Hygeia#shivan' for a quick/easy fix that won't be too visibly obvious; there's also a Shivan-reskinned GVS Nephthys support ship on FSMods, which is also used in Tides of Darkness).
