Note the uses of the word Beasts in the description of what Wild Shape does.

Beasts is a defined term; from page 2 of the Dungeon Master Basic Rules:

Beasts are nonhumanoid creatures that are a natural part of the fantasy ecology. Some of them have magical powers, but most are unintelligent and lack any society or language. Beasts include all varieties of ordinary animals, dinosaurs, and giant versions of animals.

You can Wild Shape into existing "Giant" animals

For Giant (x), if the creature meet the requirements of the Beast Shape table on page 66 and the Challenge Rating requirements on the same page, then the answer is yes, you can transform into a giant beast.

For example, a Giant Scorpion is described as

Large beast, unaligned

So it meets the Beast requirements, but its CR of 3 exceeds the maximum challenge rating of 1 for an 8th level druid. So you can't Wild Shape into a Giant Scorpion.

However, a Giant Poisonous Snake is CR ¼, so you can Wild Shape into this creature if you seen it before and are at least 2nd level.

You can't Wild Shape into swarms

As for a swarm, the answer is no. This conclusion hinges on the wording of both Wild Shape and the Swarm creature.

Wild Shape (PHB page 66) opens with

Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.

The plain reading means the druid character can shape into a single beast.

The swarm descriptions are formatted like this:

(swarm size) swarm of (creature size) beasts, (alignment)

For example a Swarm of Poisonous Snakes (DM's Basic Rules, page 46) is

Medium swarm of Tiny beasts, unaligned

So it clear that the "creature" is only treated as a single entity for the purpose of combat. It is really comprised of many creatures, whose action in concert gives the swarm its power.

My opinion is that they forgot to define or haven't finalized the definition of a swarm, which is why it doesn't appear in the list of standard creature types on page 2 of the 5e Dungeon Master's Basic Rules.

From the Monster Manual, p.165:

Goblins have an affinity for rats and wolves, raising them to serve as companions and mounts, respectively. Like rats, goblins shun sunlight and sleep underground during the day. Like wolves, they are pack hunters, made bolder by their numbers. When they hunt from the backs of wolves, goblins use hit-and-run attacks.

This is reported in the general description of Goblins. It does not mean that they can control rats and wolves like if they were using a spell like Dominate Monster (i.e. make them perform specific actions by command), but rather that they raise these beasts in order to make them friendly (like humans and dogs, or humans and horses).