PTF, SE4 Gold has a neat feature that allows modders to specify two pictures per ship class: a preferred and a second choice. If the preferred isn't found, the second choice will be used, and the second choice is one found in the default set, and looks fine IMO. As of Proportions 1.5, all new ship classes first look for a picture with a name as defined in the SE4 Image Neo-Standard. See
http://www.sandman43.fsnet.co.uk/neostand.htm for lists of ship sets that include the extra ships.
As for when the AI takes over, Fyron has the best answer, which is turn turn all (or almost all) of the ministers off in your empire settings, so your existing orders will be followed, and the AI won't go rampaging around like the nut it generally is. If you had to pick an AI file to use, though, I would use the Proportions Amon'krie one, as it is the most tailored to the mod. This would mean you would need to make a shipset with the AI files you want in it, and upload it to PBW so the server can use those files. I'm not 100% sure PBW will even do that courtesy, though - it might not bother to load custom AI files from a player's ship set - I dunno.
PvK