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 ΒΌ, 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.
The ability is monster-only
The Fire Giant Dreadnought's ability is an instance of a "specific beats general" rule. Generally, a character can only gain the benefits of a single shield. The Fire Giant Dreadnought has a specific rule stating that it can benefit from dual shields, so it can do so but this doesn't apply to any monster or character without such specific rule.
No change to the general shield rules are in the Player's Handbook Errata.
House ruling the shields is always an option, but may cause poor balance especially on lower levels when sources of AC are very limited. DnD 5e is designed with the principle of bounded accuracy (see for example this answer, meaning that AC and attack bonuses scale slowly with growing level. An extra shield will put one's AC several levels ahead in relative defense, and significantly reduce the threat level of enemies who now need to roll very high (possibly even natural 20) to score a hit in the first place.
Best Answer
The answer to questions like this always ends up depending entirely on your playstyle.
No
The gamist answer to this question is "no." The game rules are all that create the abilities and their operation, and the rules don't say it can be suppressed. You would choose the gamist answer if you desire a rules-first/RAW playstyle where the rules determine the physics of your game world.
Yes
The simulationist answer to this question is probably "yes." Real world fire beetles can control the degree of their luminosity, so it'd be a reasonable extension that giant ones do too. You would choose the simulationist answer if you focus more on the fictional game world and consider the rules to be a simplified abstraction of this that you can augment with "reality" as needed.
Maybe
The narrativist answer to this question is "maybe." You'd decide either based on what is more interesting to the plot, or based on the fact that an interested PC is asking is an opportunity to let them answer the question themselves as part of sharing the narrative responsibility for the game. You would choose this if you are more focused on the plot of the game and are comfortable with it not flowing directly from the rules or being consistent with other campaigns.
Choosing
All of these are valid playstyles with their own pros and cons outside the scope of this question, but the point is that there is no "right" answer to this question, it depends on your perspective on the game, and there is no "right" perspective. Understand your own perspective and needs, and then the answer that fits that perspective follows.
Earlier editions of D&D and the general D&D play community has leaned in specific directions on these playstyles over time, for example in 1e-2e time there were plenty of "Ecology of the Fire Beetle" type articles written from a sim point of view, while 4e was very strictly gamist with a slight amount of narrativism, but with 5e it's been left deliberately agnostic so you can use whichever style you and your players prefer fairly readily.