Well, sneaking infected into the ranks would be tough right now. Making an infected that could pass for even a short while would be a bigger project than the cannons. It would be do-able but not easy.
Well, if they only make about 3 projectile infected per cannon they can be finished by dusk. And just moving standard infected into the area in front of the buildings housing the cannons would be enough of a delay/warning since Self has never defended a building before, except its own. They'd be looking for more infected and any of the network. Plus, Self put the cannons onthe top floor (but not the roof) for the extra height due to its inexperience - for the added distance boost the height would give.