If a creature is naturally poisonous, like an aranea (MM 15-6) or a couatl (MM 37-8), can the creature freely use the feat Poison Spell (Und 51) to poison all its spells that require a melee touch attack? According to the feat, "The dose of poison used as the component [necessary to employ the feat] is expended when you cast the [now-poisoned] spell, whether or not the spell or poison successfully affects the target." Is there something that makes the aranea or couatl care that it's just expended a dose of poison, given that it has, essentially, limitless doses of poison?
From a game balance perspective, unlimited poisoning of such a creature's spells seems reasonable--such a creature's spellcasting is usually below a comparable PC's spellcasting, so adding its poison to all its melee touch attack spells isn't nearly so horrifying as it might first appear (and it forces a spellcaster into melee, which is rarely pleasant)--but I was wondering if there were a limit that I might've missed or if the definition of a dose of poison somehow prevents a creature from using, or limits a creature's ability to use, the feat Poison Spell.