...MU90 + PLG100XG.
As far as I'm aware, the MU90 cannot host PLG boards. Furthermore, unless you're simply after more polyphony, it doesn't make much sense to add an XG tone generator
to an XG tone generator.

So you say my MU50 is the same for games as PLG?
More than likely, yes. For games, you're mostly looking at use of just the capital and GS-compatible TG300B tones, which, to my knowledge, are the same across all XG tone generators (until you get to MU100 and successors, which feature both "Basic" and "MU100 Native" capital-tone maps).