the only problem with them investigating your hand is when they almost get sucked up into the gravel vac luckily I yanked the tube out as fast as possible and saved the little dumba$$Yes your 3 have taught my 4 to investigate my hand anytime it's in the tank and to generally pester the crap out of me til I give in and give treats (catfish). I mostly like it actually and yes I can pretty much lift them almost out of the water. They don't seem to mind until out so I don't take them out.
I say the more the merrier! Realistically thought atleast 3, magpie 3 mixed with my 4 no issues at all.