I really wouldnt use the IMG gunboat to be honest. While the two groups mine different things, they exist on opposite sides of sirius and any time they do meet it's probably in a competetive manner.
All of your gunboats for now should be kusari explorers. In 4.85 you can switch up to Zoner Gunboats if you want - the zoner GB's have much less firepower than other gunboat class ships, but apparently have more armor and a lot of cargo space (making them like mk2 versions of the armored transport) - which would be perfect for a trader/military hybrid faction.