That still doesn't answer my question: how does the game prioritize conflicting brick packs? For example, if two brick packs tries to use the non-default category 'Zombie', for example, but uses different icons, how does the game choose which one to use? If two servers use either of the packs exclusively, i.e. pack 1 but not pack 2 and vice versa, the players would see different icons depending on which server they visit which would be confusing as hell.
Also, substituting properly descriptive icons with randomly chosen brick icons is a terrible idea, because 1) the icon is most likely not designed to represent the contents of the entire brick pack, and 2) the icon chosen would be different depending on which brick packs you have loaded. Even worse is the way brick packs with no icons would be handled; in this case, the 'Zombie' tab would either have a single 'Z', which could mean anything from 'Zit' to 'Zebra', or have the entire category name written on it, which would make it wider than all of the tabs that have icons, and that would look awful.
The point is, your system is very poorly designed and causes unneccessary complication for add-on makers and end users alike. It's just not worth what little (subjective) gains there is for the UI design.