However, you can use any faction gun on Freelancer ID. The two main drawbacks would be getting your powercore reduced based on this chart and also if you don't gain the official owning faction's permission they wouldn't really be happy about it, and you may run into trouble with their pilots.
As for the shield, you should mainly base your choice on the type of guns your main enemies would be using. Each group has a different type of gun, and each shield offers a higher resistance against a couple of those types. You can check the shields' infocards to see which shield works better against which guns.
The main issue with codename weapons is their slow projectile speed, which makes them much harder to aim than guns with 700/750ms projectiles. They also can't be bought from any regular base, and you'd have to either loot them yourself or buy them from another player. If you choose to buy them from a player, they will cost you a good amount of cash. Other than that, it's more of each pilot's personal preference regarding which codenames they use.