- PNG is lossless but do not misunderstand that DDS is lossless. While DDS is just a container format, usually it is associated with S3 compression which is lossy, so anyone telling you to use DDS is asking you to use lossy compression.
- No games use .X as a native format, and security is but one of many reasons. Compression is not security. All games use their own proprietary format and you will be able to make your own as well. Instead of security, the reason most game companies use their own model formats is because they can control what data their model files contain. This is particularly important, since it does not make sense to keep data that your engine can not use, while at the same time it makes sense to be able to add data that your engine could use in the future. You need flexibility and thus you need your own format.
- Copyright has no meaning to anyone until sent a cease-and-desist order. It is not possible to count the ways in which Super Mario World has been violated, etc. Usually no lawsuits take place, but that does not mean ripping and stealing of assets is not taking place.