Mixing memory sizes?
You should be able to freely mix-and-match memory module capacity, up to any system/firmware maximum size limitation. I do not recall there being any requirement for "matched sets" in those days. In some cases you may need to use one socket before the other, or in other cases, put the larger module of the two in a certain socket. I'm assuming that you have already tired to consult the manual for the system, if you have it. That would be your best guidance but note that with improvement in memory manufacturing occurring at that time (late 1980's), module sizes that did not exist at the time of the system's release may also work later on. This was the case many times when IBM stated a certain maximum memory size for a system, but neglected to formally update that information when larger modules were introduced.
Are all 72-pin EDO SODIMMs the same?
There can be some things that affect compatibility, but as long as they are
functionally the same then you should be okay. As long as they were spec'ed to one of the industry-standard access time values, in other words. Modules usually have a nanosecond (nS) speed grade printed on the label (i.e. -60 or -70 for 60nS or 70nS access time). You probably will not find anything lower than 60nS for EDO memory.
Another thing to consider is that there could be some 3.3V modules (vs. 5V, which was the norm). Physically, the 3.3V modules should not be able to be plugged into a 5V slot (and vice versa) due to keying along the edge of the module. However, if you buy a module sight unseen that is supposed to be for a 5V system and it's actually a 3.3V, then you'll find out about it when you are attempting to plug it in. By that time you may not be able to return it.
Now there can be other major show-stoppers if you think that
all 72-pin modules are the same. There were options for error correction (ECC) which more than likely will not work in your computer. You can usually identify those modules by them having 9 individual 'chips' on the board (instead of
. There were also earlier-technology memory that was called Fast Page Mode (FPM) that were functionally almost the same as EDO (Extended Data Out). In some cases those FPM modules could be used in the same system that also allowed for EDO, but I don't believe that you could mix them.
As far as there being differences between countries of origin, that shouldn't be too much of a problem. When EDO was new, most everyone in the DRAM industry was making them (US, Germany, Taiwan, Japan). What you generally have to watch out for are modules being offered by small companies who were "cutting corners" by using a less-than-optimal printed circuit board design for the module or not completely testing the module after it was assembled. If you stick with the major manufacturers of that time (Micron, Siemens, Toshiba, Mitsubushi, Vitelic [was employed there for some time], Hyundai [now Hynix], Nanya; not a complete list), then you'll have a better chance of getting something that's reliable (after all this time).
I would strongly recommend that whatever memory you install in the system be fully tested before you commit to using it. You can download a memory diagnostic which will perform a fairly exhaustive test for here:
http://memtest.org/#downiso
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