Questions on alternative animal companions and animal skills

[Carolus]

In a lot of druid/ranger/animal companion guides I read (such as the one by Eggynack on this very forum, I can't post links yet since this is my first post) there are lists of alternative animal companions which are not found in the druid entry. Some of these, such as the sailsnake, contain info about how you can use it as an animal companion in its own entry. However, I also see a lot of animals listed in these guides where no such information is provided. For example, the Dire Tortoise is often mentioned as a great animal companion, but if I look at its entry (in Sandstorm, p. 151), I can't find any mention of it being a suitable animal companion. There are mutiple other examples, like the Dire Eel and Dire Elephant.
Is there some rule somewhere about which animals are suitable as companions and how to calculate their effect on the effective druid level? Or are these animals just somewhat universally accepted as animal companion alternatives by nothing more than convention?

Secondly, I am not entirely sure about the class skills for the "animal class". Am I correct in thinking that the class skills for each animal are those mentioned in its entry and thus different for each animal? For example the wolf would have Hide, Listen, Move Silently, Spot and Survival as class skills, while the ape has climb, listen and spot as class skills.

Any help would be much appreciated. :)

[eggynack]

The PHB II, page 41, has an extensive list of animal companions. If you see a companion that isn't explicitly listed as one in its entry, that's typically, maybe always, the rules citation supporting its use. As for skill points, to quote the monster manual, "All listed skills are class skills, unless the creature has a character class." Dunno that that applies if the bonus is coming strictly from a bonus external to spent skill points, but either way the entry is going to be the source for animal skills, not type.

[jdizzlean]

any animal can be an animal companion if your DM allows it. try to compare the animals on the actual and alternative lists you find to what you're looking at and that can give you a comparable power level, save some of the red tape.

of course burning a feat on i think animal cohort or whatever it is to bring the AC up by 3 lvls would mean you could just pick something at your HD and then not worry about it.

[eggynack]

any animal can be an animal companion if your DM allows it.
Well, yeah, but that's kinda a vacuously true statement. Anything can be anything if the DM allows it. All of these creatures, unless I've made an error somewhere, are just straightforwardly RAW legal, without any outside DM intervention required.

[Carolus]

The PHB II, page 41, has an extensive list of animal companions. If you see a companion that isn't explicitly listed as one in its entry, that's typically, maybe always, the rules citation supporting its use. As for skill points, to quote the monster manual, "All listed skills are class skills, unless the creature has a character class." Dunno that that applies if the bonus is coming strictly from a bonus external to spent skill points, but either way the entry is going to be the source for animal skills, not type.
Much obliged! I hadn't realy seen that list yet, but it really does give a lot more options. Your guide (really great guide by the way :)) seems to have listed the strongest options, but it does give some interesting flavourfull options.