No, they really aren't interchangeable. sodium lactate is a additive, it has no correlation to the amount of lye used while beeswax has to be added as part of the oils to be saponified.
Beeswax also will alter the feel and lather of a soap while SL really doesn't. Another thing that helps harden soap is a little salt. Use 1 tsp per pound of oils used in a recipe. Dissolve it in your water before adding the lye.