materials in gallery build up

Everything you wish to do is possible.

1. There isn't a way to not having a created material appear in the gallery?
Answer: Yes there is. Select your model, object and then select Material in the Properties Toolbox top left corner of Swift 3D interface. Here you can edit the objects material the same way as you would do for a material in the library, only that this material will not create a new material in the Material Library (gallery).

2. Or at least have it only appear in the gallery when opening the swift file it was created in?
Answer: The Material Library is not dynamic to your Swift 3D scene. However if you create a scene with various materials on objects, when you save that scene all materials applied to your scene will be saved in that file. Therefore you can open that file at work and the materials will still be accessible via the Properties Toolbox under Materials for each object.

3. Now when I created 3d artwork with several custom materials, they remain in the gallery even when starting a new swift file. Working on several files (e.g. for various clients) this build up of materials in the gallery window can get pretty big.
Answer: You can create your own Categories within the Material Library to keep the Library organised. You can also delete Categories or Material from the Library.

4. While we're on the subject of materials: it also isn't possible to export materials? So I could open the swift file at the office with the materials in the gallery I created at home?
Answer: I sort of covered this in 2. above, however you can export Materials or even whole Material Categories. Go to your Program Files>Electric Rain>Swift 3D>Version X.00>Materials directory and you can copy a Category folder (containing numerous Materials) or just copy the individual Material files .t3m These files can then simply be placed in the same folder on another machine using Swift 3D. Open Swift 3D and the Materials/Category will appear in the Material Library ready for use.