I am new to this site, so hello to all ..
DJs are performers, and as I do live sound as well, I can tell you I wouldn't want any performer to touch my equipment. Any reasonable club (and I will differentiate this from mobile DJs) should have a system in place where a performer can plug in and expect that given a signal from his equipment, that all the downstream components are already in place and tuned to the club's environment. SO speakers, amps, room eq, delays, etc. are all set for that performance venue.
If the venue provides a mixer as well, then inputs should be labeled properly and jacks provided in a convenient and well labeled area. IF not, then the venue should have it clear in their contracts that it is either the promoter or the performers responsibility for all of that. It certainly doesn't make sense to have part of that stream in place (a couple of hanging speakers, some electronics hanging around) .. you are begging for a problem with compatibility and will end up with the issues posed by the OP. This is the common sense needed, not certifying DJs for activities they shouldn't really be handling.