The common archerfish species get to at least 12 cm in captivity, and two of the three species around 15-20 cm in captivity. Wild fish supposedly get much bigger, but that doesn't happen often in aquaria.
Monos likewise seem to get to about 15 cm in home aquaria, though specimens in public aquaria often seem to be closer to 20 cm in length.
Knight gobies reach a maximum length of about 7-8 cm.
Mixing any fish with Figure-8 puffers is a gamble; that said, many people do, and things work out. But do have a Plan B up your sleeve in case your specimen turns out to be a bit nippy.
Cheers, Neale