Can't really help ya with the spider, but you need to give the slug [COMMON_DOMESTIC] for it to show up on the embark screen.
[CREATURE:CANNON_SLUG]
[COMMON_DOMESTIC] [DESCRIPTION:A huge blue slug-like creature capable of shooting chunks of obsidian.]
[NAME:cannon slug:cannon slugs:cannon slug]
[CASTE_NAME:cannon slug:cannon slugs:cannon slug]
[CREATURE_TILE:'~'][COLOR:1:0:0]
[PETVALUE:100]
[POPULATION_NUMBER:25:50]
[SPEED:1500]
[PREFSTRING:blue slime]
[MATERIAL_BREATH_ATTACK:INORGANIC:OBSIDIAN:LIQUID_GLOB]
[BODY:BASIC_1PARTBODY_THOUGHT]
[USE_MATERIAL_TEMPLATE:SKIN:SKIN_TEMPLATE]
[USE_MATERIAL_TEMPLATE:FLUID:CREATURE_EXTRACT_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen cannon slug slime]
[STATE_NAME_ADJ:ALL_SOLID:frozen cannon slug slime]
[STATE_NAME:LIQUID:cannon slug slime]
[STATE_NAME_ADJ:LIQUID:cannon slug slime]
[STATE_NAME:GAS:boiling cannon slug slime]
[STATE_NAME_ADJ:GAS:boiling cannon slug slime]
[STATE_COLOR:ALL:BLUE]
[PREFIX:NONE]
[SYNDROME]
[SYN_NAME:slug syndrome]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_IMMUNE_CREATURE:CANNON_SLUG:ALL]
[SYN_IMMUNE_CREATURE:DWARF:ALL]
[SYN_CONTACT]
[CE_PAIN:SEV:5:PROB:100:LOCALIZED:VASCULAR_ONLY:RESISTABLE:START:50:PEAK:1000:END:2000]
[TISSUE:SKIN]
[TISSUE_NAME:skin:NP]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:SKIN]
[TISSUE_MAT_STATE:SOLID]
[STRUCTURAL]
[RELATIVE_THICKNESS:1]
[CONNECTS]
[TISSUE_SHAPE:LAYER]
[TISSUE:FLUID]
[TISSUE_NAME:fluid:NP]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:FLUID]
[TISSUE_MAT_STATE:LIQUID]
[MUSCULAR]
[FUNCTIONAL]
[STRUCTURAL]
[RELATIVE_THICKNESS:100]
[CONNECTS]
[TISSUE_LEAKS]
[TISSUE_SHAPE:LAYER]
[TISSUE_LAYER:BY_CATEGORY:ALL:SKIN]
[TISSUE_LAYER_UNDER:BY_CATEGORY:ALL:FLUID]
[SECRETION:LOCAL_CREATURE_MAT:FLUID:LIQUID:BY_CATEGORY:ALL:SKIN]
[SPEED:1200]
[BODY_SIZE:0:0:60000]
[NO_SLEEP]
[NO_DIZZINESS]
[NO_FEVERS]
[CLUSTER_NUMBER:1:3]
[SET_TL_GROUP:BY_CATEGORY:ALL:SKIN]
[TL_COLOR_MODIFIER:blue:1]
[TLCM_NOUN:skin:SINGULAR]
___
Also, thanks I'll try the [PET] thing.