For the patches error, you just need to give your “Make modularsynthpreset” node an empty dummy-array. If you extend the node, it is pretty much at the bottom. Drag a blue line from it and type in node “make array”. Will solve it!
You can play chords from one instrument on a single synth. You want different presets for different instruments, and they need to run on their own separate synthesizers if you want them playing at the same time. To understand and wield all the parameters, and also to be able to separate engine-terms from sound-terms, I highly advise learning generic sound synthesis. If you learn one synthesizer, you will also understand all others + unreals new synth automagically. I highly recommend http://www.syntorial.com/. Maybe find some documentaries on the early days of synthesis, as you will learn a lot there too, while being entertained 