I'm sorry I've been reading Yandex Slovari all wrong, could I still get a pass on whether I'm thinking about it the right way in the first part

-and also what does the fact that впустил is imperfective and шмыгнул is perfective mean? He dashed to the door "letting" in the dust? If they were both perfective would it be.. he dashed and then let in, in two seperate steps?