Scenario 1: Loki joins. It will ambiguous who is stronger between Zoro and Loki until next arc when Zoro clearly is shown to be stronger (lex Robin post Alabasta)
Scenario 2: Loki joins later. When he finally joins Zoro will be shown to be stronger (lex Jinbei post Zou, perhaps Yamato as well?).
Scenario 3: Loki will be a temporary ally like Law so he is outside of the crews power scaling, to the WG he might even be equal to Luffy.
In no scenario will Loki take the #2 spot.