Targeting a Swarm with Polymorph

Can you target a swarm with Polymorph to turn it into a single creature?

RAW? Yes, there’s nothing preventing you from doing it, the stack of immunities that one gets doesn’t prevent Polymorph from being used on it. RAI, I’m not sure, I think they talked about this once.

Quote from Cramissioner >>

Can you target a swarm with Polymorph to turn it into a single creature?

This is an area where they've issued some rulings that aren't super consistent.

You pretty much have to treat a swarm as an individual creature most of the time or a lot of the rules break down around them. If a swarm doesn't count as a single creature for targeting purposes, then there are a ton of spells and other effects that you can't use on the swarm as a whole at all, which makes them unreasonably resilient in a way that doesn't really make sense. 

But we've also had things like this old Sage Advice question (which says that Conjure Animals can't summon a swarm because it's not an individual creature) and various rulings that Druids can't use Wild Shape to turn into a swarm because it's not an individual Beast. These suggest that the intent is for a swarm to only count as an individual creature sometimes, but where exactly the line is drawn has never really been specified.

In practice, a swarm is rarely a significant threat to someone who can cast Polymorph — I've never seen one with a CR higher than 5 — so I don't think it's going to break anything if it's allowed.

Quote from wagnarokkr >>

[...] various rulings that Druids can't use Wild Shape to turn into a swarm because it's not an individual Beast. [...]

 Yeah, for example, this is from the Dev:

@TStrek22 can druids wild shape into swarms?
@JeremyECrawford Wild Shape lets you transform into a single beast. A swarm is a collection of beasts, not one.

EDIT: adding the quote.

Thank you for digging that up, I did not feel like descending into the nine hells of twitter last night.

A swarm is treated as a creature with a list of resistances and immunities. Given that swarms have immunity to being petrified, it's probably intended for them to be immune to polymorph effects, but as written they aren't.

I'm not a big fan of the swarm rules, I prefer something like "Swarm Defense: a swarm is resistant to damage and immune to conditions from effects that do not target an area".