Can my PMP powered mixer connect to just one passive loudspeaker?
Yes. All of the Behringer PMP series of mixers can be used without connecting speakers, or with just one loudspeaker connected. You are not required to connect to both outputs.
In general, if an amplifier (either valve or solid state) has an output transformer, then a speaker/load must be connected. If there is no output transformer the amplifier can be used without a load connected.