Answer to "Can I connect 12v 18amh battery?"The different AMH ratings should not be an issue, except for run time not being equal. As long as they are both 12VDC, you should be ok.
Answer to "2.8 kit water leaking through db25"I would look for the leak to be seeping through a gap between the wire terminals. You may be able to reseal this area by more epoxy and using a syringe to get it where you want. If this fails, I would order another cable and try again, being more particular to get more epoxy deep down into the DB25. You may find that thinning the epoxy or going heavier on the superglue for the nooks and crannies will seal it up.