So are there "what you see is what you get" rules? An if so do people generally ignore them
It depends on who you expect to play. If you're not going to play in ITS tournaments you can talk with your opponent first. Otherwise you will be expected to have the correct mini where available, with some exceptions:
- Not all unit and equipment options are available yet. If not, proxies are allowable.
- Some options are not reflected on the mini, so proxies are acceptable.
For example on the second point - in most cases the medic or engineer option of a unit doesn't have anything on the mini that distinguishes it. So you could proxy this using a mini of the same unit type with the same weapon.
The biggest concern generally is that the weapons displayed are accurate. So you shouldn't use a model with a Combi-rifle to represent one with an HMG for example. In some cases the model sculpt isn't holding a weapon so this allows options. Some weapons are never shown on the models, such as the different pistols or close combat weapons.