Hello, Franky.
All of the Macs with built-in serial ports have so-called "real" serial ports, which appear to software as a "Modem" or "Printer" port. A serial port is not a so-called "real" port when it rides over another type of interface such as USB, as is required on Blue+White Power Macs, Bronze/Lombard PowerBooks, or newer machines.
ADB is a different type of interface, not a "serial port" in the conventional sense. It is mostly only used for input devices (mice, keyboards, tablets, etc) and there are no MIDI interfaces I know of that use this port.
AppleShare (file sharing) on older machines used a networking architecture called AppleTalk/LocalTalk/EtherTalk, which is not supported anymore. The file sharing protocol was ported to the TCP/IP architecture in the late 1990s under MacOS 8/9, and can communicate with systems as recent as Catalina. So if you want to share folders from one machine to another, you have to plan carefully. But there are standard Internet file transfer applications for all systems, such as FTP, which overall is more appropriate for transferring samples etc. There are FTP tools for System 6 and it is built in to the latest systems, so no problems there.
As far as choosing between traditional "Zilog 85C30 SCC" ports and GeoPorts, I don't think there is much if any practical difference in compatibility. A GeoPort just has an extra pin for +5V (which isn't used), and a DMA engine (that isn't used). They both support the external 1 MHz clock that comes in from the MIDI interface to set the MIDI transfer rate (1 MHz divided by 32 = 31250 bits per second). More pressing compatibility problems for older software are probably 68k/PPC emulation related.