Another aspect of using latex inner tubes that you have to keep in mind is that you also have to make sure not to stretch the tyre bead when mounting it.
Doing so may lead to inexplicable sudden explosions of the inner tube because it has found a pathway to leave its container...
And yes, 18-20 inner tubes only belong in a tyre of the corresponding size, i.e. from 18 to 20mm wide.
Not sure if this has been mentioned but when you do patch a latex inner tube, just degrease the punctured area with rubbing alcohol and use round latex patches you cut out from an old inner tube. No butyl and no squared edges please.