Is it advisable to apply an alkyd over an existing latex?

No, it isn’t. The alkyd layer will get hard and brittle as it ages, but the latex underneath will remain flexible. As time passes, the latex layer will flex and move with the expansion and contraction of the substrate more than the alkyd topcoat will. This will compromise the adhesion of the alkyd to the underlying latex and cause the alkyd topcoat to peel.