yeah i don't think that a cow would fit in a 45, maybe there are small kinds, IDK.
tobies are nice and full of personality, a pair of maroon clowns or another large clown species would be cool.
dwarf lions are pretty cool as well but don't do a whole lot, even by lionfish standards. hawks are nice as are smaller wrasses like six lines and twinspots
if going with a mid agresive tank, the larger damsels, velvets, sgt. majors, dominos, are good and can be pretty "pet fish" like.