If you got nationalized gunners you'll just have to use the nation guns. Nationalized gunners can't use neut guns but the later tiers of the neut's can use low level national guns

What have you spent your credits on?  You can sell your neutral ships - but make sure you sell things like armor/components separately.

You can still use your neutral ships with national sailors as mentioned.  The Bridge officer is fine as long as the level qualifies.  Regarding weapons.  You can rearm the ship with those that match your nation.

Feel free to post any other questions you have.

Play blitz and change the guns on the frigate to the nationality you chose.

make sure you class all your sailors to the same nation.

and a couple of the lower lvl crews that you havent nationalized yet are probably still able to use the guns (doesnt matter if they dont have gunner stats at that lvl).

ask around in blitz rooms, there's probably people willing to help you out with this kinda stuff, we've all been there at some point ;)