Can I summon a flail snail or invisible stalker with Conjure Elemental?

Question

The conjure elemental spell says:

You call forth an elemental servant. Choose an area of air, earth, fire, or water that fills a 10-foot cube within range. An elemental of challenge rating 5 or lower appropriate to the area you chose appears in an unoccupied space within 10 feet of it. For example, a fire elemental emerges from a bonfire, and an earth elemental rises up from the ground. The elemental disappears when it drops to 0 hit points or when the spell ends.

Does this mean a creature can not be summoned unless it belongs to one of the four elements? For example, can I summon a flail snail or invisible stalker with conjure elemental?

Answer 1

Flail Snail and Invisible Stalker are both valid choices for conjure elemental

Choose an area of air, earth, fire, or water that fills a 10-foot cube within range. An elemental of challenge rating 5 or lower appropriate to the area you chose appears ...

Firstly, the Flail Snail is CR 3 so it is a valid choice for this spell. The Invisible Stalker is CR 6, which would be invalid, but the spell can be upcast to increase the CR range, so if the spell is cast with a 6th level spell slot or higher, the Invisible Stalker then becomes a valid choice too.

From the PHB, pg. 225:

At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the challenge rating increases by 1 for each slot level above 5th.

Secondly, both of these creatures are "elementals" (which is one of the creature types like "humanoid" or "beast"); this is what the spell is referring to when it says:

An elemental of challenge rating 5 or lower

not just those that are called "Elemental" such as "Fire Elemental", etc.

This creature type is outlined in the Monster Manual, pg. 6:

Elementals are creatures native to the elemental planes. Some creatures of this type are little more than animate masses of their respective elements, including the creatures simply called elementals. Others have biological forms infused with elemental energy. The races of genies, including djinn and efreet, form the most important civilizations on the elemental planes. Other elemental creatures include azers, invisible stalkers, and water weirds.

Also, both of the creatures you mentioned are each associated with one of the four elements. Flail Snails are associated with earth (Volo's Guide to Monsters, pg. 144):

A flail snail is a creature of elemental earth ...

And Invisible Stalkers are associated with air (Monster Manual, pg. 192):

An invisible stalker is an air elemental ...

Answer 2

The Sage Advice explanation is that the DM decides which creatures appear when a summoning spell is cast. See:

How do you determine which beast appears during Conjure Animals?

In short: the player chooses the elemental area, and by that, the general type of elemental. The DM describes what specifically appears.

If you are the DM, the wording of this spell does not seem to preclude the more-exotic elementals, so, as long as that type would be appropriate for the elemental area the player selects, and that's what you think would be best, go for it.

In practice, many DMs let the player say what they want to appear. But there's no reason they'd have to do that if the player using the spell in a way that feels over the top.