For starters, definitely get a proper speaker cable! A instrument cable wont do, because the shielding around the central wire cannot cope with the current that flows through the cable. You want something like 2-core mains cable (2 cores, unshielded) with jacks on either end, shouldnt cost more than a few bucks
It sounds like you're pushing the jack in far enough to make a connection, but not far enough to trigger the switching part of the socket (ie the bit that switches off the internal speaker). Although this isnt really advisable (the jack could get pulled out easily, which wouldnt be good), it wont necessarily damage the amp..
You cant really say whether this will damage it or not without knowing how the speaker outputs are wired - if theyre in parallel (most likely), then youre running a load of 4ohms. Seeing as the ext. speaker jack specifies 8ohm minimum, this could cause damage (hard to tell for sure, have a look in the manual for some data about the output transformer)
Best bet is to get in touch with Laney and ask them.. If theyre not too keen on you "cheating" their system then try asking how the speaker jacks are wired (series/parallel), and if the output transformer can cope with a load of 4ohms