This is only a partial answer, and I suspect a couple might be wrong, but I think the theme is correct. The couplets refer to

>!Signs of the Zodiac

A

My 11 from 12 sealed Atalanta's fate

But what did my 7 expect me to crate?


>! Taurus (the bull). As explained by Kingrames, The 11th labour of Hercules was to steal the Apples of the Hesperides, which were later used by a suitor of Atalanta to beat her in a race and force her hand in marriage. The 7th labour was the Cretan **Bull**

B

With a merged name between us and unusual style

We have twice tried to win for the Emerald Isle.

>! Gemini (the twins), with a reference to Jedward, twins who represented Ireland in Eurovision twice.

C

Commonly seen in a number of sports

Frequently 3 can be worn in support.

>! Leo (the lion). As in Detroit Lions, or the British and Irish Lions. England supporters wear three lions on their shirt.

D: 

Well defended from predators except when I roam

From second-hand home to second-hand home.

>! This is Cancer (the crab)

E

There's nothing from nothing and nobody's out

Other than those when we last swapped about.

>! This is Virgo, the maiden. In cricket, a maiden over is one where no runs are scored, and also no wickets taken (otherwise it would be a wicket maiden). At the end of each over the fielding team has to swap bowlers (credit to @'ff)

F: 

When cannons appeared we were quickly retired,

But the force like to use us where force is required.

>! This is Aries (the ram). Cannons replaced Rams as a siege weapon, and the police use a ram (thanks to APrough)

G

Between Shakespeare and Sauce for 65 years

Our voices are heard by myriad ears.

>! This is Sagittarius, the Archer. The Archers is a BBC radio soap opera, whose Pilot episode was broadcast in 1950. Not sure about Shakespeare and the Sauce though.

H

Digitless pets and fodder alike

Beware where you crawl for some of us bite.

>! This is Pisces (Fish)

I

A means of propulsion for a friend of the hulk

If someone gets yours you are likely to sulk.

>! This is Capricorn, the goat. Somebody can "Get your goat". The friend of the hulk is Thor, who has a chariot pulled by war goats (thanks to @Bishop)

J

No further than Sheppey can our like be found

As our 4 pairs of legs like the warmest of ground.

>! This should be Scorpio (the Scorpion) which has 8 legs and likes warm climates. According to wikipedia "The isle [of Sheppey] is noted as the northern-most place to have an established scorpion population".

K

How big or how heavy? On the hull of a boat

Sometimes reptilian we hold all your notes.

>! This is Libra (the Scales). Scales can weigh, reptiles have scales and there are musical scales.