There are 2 significant GUI issues with exceeding the maximum character cap:
- Beyond 4 characters (Warden + 3 companions), additional characters do not get portraits. This makes it harder to select them, as well as to keep track of their health & mana during combat.
- Beyond 4 characters, additional characters do get quickbars, but unfortunately they are grayed out. This means this handy tool can't be used to quickly cast spells or activate talents/skills. Neither does pressing numerical keys or pressing buttons in the skill, talent or spell screens work. In fact there is no direct way to access their abilities so you have no ability to micro-manage these characters. All you can do directly is command them to move and/or attack. The one thing that does work is tactics. You can set up their tactics options and they will faithfully follow them. This isn't so bad as long as you keep mages or other characters you desire to micromanage among the three companions with portraits. Characters that can be reliably automated tactically can then be added in as extras.
While much of the game is flexible enough to be modified by users, unfortunately the GUI is one aspect that doesn't share that flexibility, at least not currently. So these problems must be lived with by those wishing to run with extra characters.
Well sort of, there is one trick can be used that is cleverly taken advantage of by Dheuster's extra dog slot mod. Instead of placing them in a character slot, they can be placed in a summoned creature slot, providing characters with a small status portrait and making it possible to use their skills/talents/spells directly. This would require additional scripting to add other companions in a similar manner. It isn't a perfect solution, but it's probably the best that is currently available given the constraints of the current GUI. Nevertheless, as far as I can tell nobody has written a mod to accomplish this as of yet. So for the moment everyone appears to be stuck with zz_addparty or their own personally scripted version of it if extra characters beyond Dog are desired.