What's the result? Wrong sprite, no sprite at all, or errors?
I think I have an idea what the problem is - on a closer look, monster_get_battlesprite takes an AMP ID argument (AMP is short for Active Monster Party, an AMP ID is a reference to the data of one particular monster, like your Pikachu named Fred) but not all of the cases uses the AMP, some use the species directly and bypasses AMP:
(red underline = AMP ID, blue arrow = uses species directly, darked out = no need to care about this for now)
The easiest way to solve this probably would be to have a new "mon_species_get_sprite" function and move your gender-sprite logic to that, and then monster_get_battlesprite would just read the species and then return the mon_species_get_sprite result of that species. Now the lines that use the species directly can use mon_species_get_sprite (using the original species variables we used in the array accesses) and the ones using the AMP ID can use monster_get_battlesprite.