Hi AllusionAtelier,
Have you enabled "Exceed Party Limit?" in the Plugin Manager -> Summon? Unfortunately, this feature has a higher potential to cause trouble with other plugins due to how RPG Maker is programmed. As this plugin is entirely free, I'm afraid I will no longer maintain this feature.
What could work for you is to make a tiny plugin with any name, put it below MK_Summon, and insert this:
Window_BattleStatus.prototype.maxCols = function() {
return 1;
}