Map add-ons are simply other maps that do not cause any harm to each other when installed.They do not cause any gameplay changes, except for certain visual upgrades for a better experience. The visual mods have special characteristics from normal mods in that they are compatible with one another hence they will do no harm to each other once they are installed.Once you have placed the mods in the new folder, you 500 mission limit should be lifted.
Since you might refer to them at a later time, save them by creating another GameData folder which you will now call addon _ inactive. The readme document that was packaged with the download may contain additional instructions. Keeping Your Game Safe When Installing Mods
