I'd like to know if anybody out there has ever done a system that allows a caster to cast and control more than one summoned monster at a time, and if they wouldn't mind working with me to adapt that system to a module that I'm currently working on.
What Kalbaern said. You technically cannot have more than one summon but you can "fake it."
In addition to this, I'm also looking for any systems that allow a caster to choose between a variety of summoned monsters instead of the same one each time. I've seen this as well, either as an option in rest menus or set by the unique powers of certain inventory items. Can I get the details on how this was done?
Simple way would be to make an item the PC can't get rid of (cursed) and store information on that. Let's say you want to let the user choose what type of elemental to summon. Have 0 represent Air, 1 Earth, 2 Fire, 3 Water. Can have the item have a unique power to start a conversation and then in the conversation give dialogue options for each type. When the user selects a type, set a local integer based on their choice (for example, could have the int be called "summontype" and if they select Earth from dialogue then you set the value to 1).
Finally, when the user casts the summon spell check the int on the item and summon the appropriate type.