aaaahhh! Nice retro synth!
What do you mean exactly with module? If you have in mind to create a plugin which emulates the Korg01 and you want to use it like a GM synth, so you have the possibility to use samples/soundfonts. Sample-Sets are supported by fluidsynth and linuxsampler (both are called engines in Zynthian-speech). You can also create a LV2 plugin with a builder-script based on fluidsynth (see https://discourse.zynthian.org/t/mod-ui-pedal-for-sfz-input/543/11) and use this plugin inside MOD-UI (also a engine - but very flexible by using several kinds of plugins). FluidPlug needs a SF2 soundfont. To create SF2 soundfonts you can use Polyphone.
Now for your MIDI inputs: On software side you can have as much MIDI-inputs as your engine supports. On hardware side Zynthian has one MIDI input and can use several more via USB-MIDI, which must be routed to the engines/plugins.
Hm - I don't understand your question... Zynthian has a stereo output. Perhaps you can add more Audio-Outs via USB but you have to check this on your own. I think this will be not easy and you will get much more latency.
We (my band) are using several plugins via LV2 (Dexed, setBfreak, FluidPlug, ...) on a Zynthian live. The Zynthian is directly connected to a mixer.
This depends on the engine, the preset (how many plugins) and the data itself: If you have a huge SF2 the loading of the fluidsynth takes only a few seconds (perhaps 1-3s?) and loading of a big SF2 takes about 30 seconds? If you use MOD-UI as engine and your pedalboard has 20 plugins this will also take some time to load - especially if you use inside the plugins bigger SF2 soundfonts... you will have to test this by yourself - sorry. Have in mind: normaly Zynthian loads all from SD card - this is not as fast as HDD/SSD! Fast live-switching (inside a song) is AFAIK not really working, but you can organize your sounds for being in "standby" and switching between them.
What do you mean with "package"? Currently the web interface has no way to upload sample files (WAVs, SF2, SFZ, GIG, Akai,...). This will come hopefully soon
As I wrote: sample players supported are Fluidsynth (as plugin and engine) and Linuxsampler (currently only as engine). Take a look at both (Linuxsampler / Fluidsynth) to get an idea of what they can do. Perhaps take a look a MOD-Duo. This one is nice for musicians. It uses only LV2 plugins but they are creating the MOD-UI as OpenSource. Most of the plugins used in Zynthian will also work on MOD-Duo.
Hm - what do you mean exactly? If you have a SF2 soundfont, you can load it into a plugin or engine. You can start several plugins/engines parallel but have to create a MIDI routing (can be done with plugins).
Hope this helps a little bit.