Hm, let's see... you seem to have made a lot of programming typos, so to speak. For example, you misnamed the Sandshrew and Sandslash female castes [both castes are called [CASTE:SANDSHREW_MALE] and [CASTE:SANDSLASH_MALE], respectively) (two castes of the same name would probably cause all kinds of problems, if the game doesn't crash outright after attempting to populate the Sandshrew) and put it on the wrong line (I don't know what effect that has, if any, but it's there). You also misnamed the Sandslash female caste, but that's on the right line.
Syntax for the SYN_IMMUNE_CREATURE (and related tags) is simply SYN_IMMUNE_CREATURE:creature:caste. In other words, the second slot contains the CREATURE tag (which I assume is something on the order of POKEMON, since the CREATURE tag is omitted from all the pokemon raws I've gotten my hands on so far). The third slot contains the CASTE tag. Thus, for example, the tag [SYN_IMMUNE_CREATURE:GASTLY:ALL] would ensure that all [CREATURE:GASTLY] castes are immune to the syndrome it's attached to, while the tag [SYN_IMMUNE_CREATURE:GASTLY:GASTLY_FEMALE] would make it so that only [CREATURE:GASTLY] of the caste GASTLY_FEMALE are immune to the syndrome.
You have several grammar mistakes in the descriptions, and I'm an advocate of plural Gastly (that is, I believe that 'Gastly' is both singular and plural, like 'moose'), but those are relatively minor quibbles.
In case you need it, more information on syndromes is available
here. The magmawiki has a lot of helpful information for general modding, as well- I practically refer to it step-by-step.

The text file that defines dwarves in vanilla DF (creature_standard.txt) has some nice basic information on how to format creature raws, as well. I don't know if you really need any of this or just made some typos because you were working too fast or something, but I'm, uh, just throwing it out there. Just in case, y'know...
...please don't hurt me.@Monk: Wouldn't it be a good idea to make up creature classes, though? Seeing as Poison-types are supposed to be immune to all poisons, you can create the POISON_TYPE creature class and add it to all the Poison-type pokemon, then make it so that all Poison-type attacks have SYN_IMMUNE_CLASS:POISON_TYPE or something? Same with Steel-type. You could also apply that to other types- making Ground-type immune to Electric-type attacks and things like that. That way, you wouldn't have to add every pokemon that's immune to a syndrome in the syndrome, especially if you ever plan on expanding past the first 151. Of course, that would make all pokemon immune to the poison of regular animals, so you'd either have to overhaul the creature classes of all the normal animals unless you plan on replacing them (the normal animals) entirely. (Unless you can give one creature more than one creature class, but that sounds like a recipe for disaster...). I'm also assuming that you plan on adhering to the canon type advantages/disadvantages, which might not be correct. So nevermind. I hope the idea helps, though.