Small players can excel in the NHL. It's harder for a defenseman though, and the few that do make it are far better skaters than Almquist is. As it stands now, he'd be overmatched in his own end and offensively he wouldn't be able to win enough puck battles to be effective enough to compensate for the poor defense.
People who are saying he'd be better than anyone the Wings have now, or Sproul, Marchenko, or Ouellet, are the "grass is greener" crowd. The same ones who said that Ericsson was going to be the Swedish Pronger, and every hot prospect was already better than half the roster before they played more than a dozen NHL games, and any given free agent would "obviously" be good here, until that player gets here and we see his flaws. If Almquist was on the roster next year, everyone would be screaming to get rid of him by Thanksgiving.