I set the <iAppearance/> value for KAKTUS to 10000, and generated a huge map on C2C_PlanetGenerator_0_68, very low huminty, Extremely hot, one big continent low ocean , thus lots and lots of desert...
100% of scrub and desert was filled with cactus (if they weren't filled by some other feature)
iAppearance of 10000 = 100.00% chance of appearing on a tile during map generation. A value of 1 equals a 0.01% chance of the feature appearing on a tile during map generation.
Pick a % frequency value you want cactus to show up at and change the iAppearance value for it, and you can remove cactus generation from the post-map script.
You also have the same problem with Kelp, Reef, and Sea-Grass. They all have an iAppearance of 0 therefore a a zero chance of appearing during map generation.
Coral had an iAppearance of 100 (which is really 1% chance of appearance. You can probably get rid of your reef/coral/kelp etc post-map script stuff also by fixing these appearance values.
Also you have an iGrowth and and an iDissapearance of 100 for both Kelp and Sea-grass. If I recall how these tag work correctly (its been a long while so forgive me if I am wrong) first with an Appearance of 0, there is no initial set to grow from. Second it will mean that those features random grow from and appear or disappear on the map during play. Is this desired? (I would not want them to disappear) If these values are divided by 100 like iAppearance we are only talking about a 1% chance per turn, per feature if it works like that, so at present it is very easy to not ever notice. iGrowth rates should probably be more on par w/ forest and jungle (16 and 20 respectively)
If we had a bRequiresCoast for features, you might be able to do your mangrove w/out the post map-script also.
Sea Lion & Seal and Walrus do not show up because they have a placement order of -1. However, you may want to keep Walrus/Seal placement in the post-map script for doing your coastal checks, unless a bRequiresCoast tag was actually put in, but I don't think it ever was. If you do a placement order of -1 for other bonus you want to hijack and control only via the post-map script, then also assign a placement order of -1 to them, or you get the occasional "in the wrong area" bonus for things like Llama and what not, if you are not infact automatically removing all found instances of them already in the post-map script.