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 ...
Best Answer
It's unclear by the rules as written. Flavor might suggest yes. Game balance strongly suggests yes.
I'm not aware of anything written in the rules that governs summoned creatures at this level of detail, which would mean this is an issue for the DM's discretion.
However, the spell description is suggestive:
Emphasis mine.
There are a few elements which seem relevant:
These summoned creatures are fundamentally not quite the beasts they resemble. There is no Giant Poisonous Snake conjured by the spell, only a fey spirit which takes the form of one. It's not a snake, it's a snake-resembling spirit. Extending that logic, there is no Serpent Venom either, properly speaking, but instead a specific manifestation of the fey spirit which is shaped by the spell.
I'm not sure how official the following interpretation is (I can't find a citation), but it's worked well as a houserule for me: a "beast" summoned by this spell is a blob of fey energy which, when brought to the Material Plane by the spell, precisely resembles the desired beast. In nearly every way (save for creature type, which is automatically fey) the fey spirit is indistinguishable from the beast it resembles.
So its venom still works the same way as in a mundane Giant Poisonous Snake, but it's not venom made from mundane elements-- it's the fey spirit exactly imitating the venom. No fey spirit, no venom, so when the spell ends and the spirit departs there is nothing remaining to comprise the Giant Poisonous Snake or any portion thereof.
There is no divisibility in the spell description. Since it doesn't describe anything about any portion of a beast remaining, there isn't a strong basis for suggesting that the spell does more than it says and leaves portions behind.
I suppose you could argue that a portion of a creature is not generally a creature but an object, and therefore could remain, but that's also for the DM to adjudicate. Creature body parts aren't statted, so it's not clear to me if they would immediately have an HP of 0 or couldn't be evaluated that way at all.
It also brings up complications in other scenarios, like summoning a Giant Boar and cutting off portions to use as food. Also clever, but it steps all over other spells like Create Food and Water.
Serpent Venom is a priced item, and at 200 gold per dose isn't particularly cheap. It's also weightless and doesn't expire or become less effective. It's not trivial to harvest, but far from impossible-- even a character not specialized into Nature or poisoning can easily get enough of a bonus to make the harvests much easier.
Taken together those properties mean that someone with access to the spell will be able to quickly produce large quantities of the venom for free (there are no material components to the spell), and carry an unlimited amount of it at a time. Adventuring becomes a money-losing career at that point.
This has balance implications for combat (that the venom is fairly expensive suggests that it's not expected to be available for every attack) and completely ruins any economic balance that might otherwise exist. If the character can cast the spell once per day, they can summon eight of these snakes, and get up to eight doses of Serpent Venom every day.
Each effort to harvest takes 1d6 minutes and the character has to make a DC 20 Intelligence (Nature) check. So success isn't guaranteed, but with eight attempts available each day even a modest bonus from INT + Nature + (Proficiency, if proficient in Nature or with a Poisoner's Kit) gives good odds of extracting multiple doses each day. (Harvesting rules from DMG, Chapter 8, under the Poisons heading).
That's up to 1600 gold's worth of goods, obtained in less than an hour, with very little risk (the harvester could fail their CON save and take 3d6 damage, but that's a manageable amount in a situation with no other health risks).
Subjective opinion as a DM:
It's a clever plan, and I like to reward those. But I would be wary of abuse, particularly if your character has good bonuses for the harvesting check, and so would either impose some homebrewed limitations or would be very ready to shut the scheme down by DM fiat if it seemed like it was becoming a problem. I would almost certainly decide that this plan arbitrarily cannot be carried out during downtime, as that would render even the cost of using a spell slot very cheap.
I also don't like making certain character choices irrelevant-- if someone in your party took Create Food and Drink as a spell they know, they chose it over some other spell they could have taken. Having your PC suddenly make that spell less distinctive and significant for reasons external to the written rules may not be fun for such a player.
Because this application of the spell does that, at least somewhat, I would tend towards ruling that the spell (RAW) can't be interpreted this way without adding some additional homebrew elements. And once we're into homebrew territory the text of the spell no longer constrains it at all, and this becomes a question for the DM rather than RPG.SE (or at least another, related RPG.SE question!).