So with a little help from some friends in the comments, we have the theme and the answers to the couplets.
But still not which of the 8 calls us home this July? Any ideas? And why 8 and not 11 or more?
The couplets refer to
11 of the 12 signs of the Zodiac. But there are only 11 couplets, and the missing sign is Aquarius (the water carrier).
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
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.
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.
Well defended from predators except when I roam
From second-hand home to second-hand home.
This is Cancer (the crab)
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 @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)
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. The series takes place in the fictional county of Borsetshire, which (according to wikipedia) is situated between the (in reality contiguous) counties of Worcestershire and Warwickshire. The town of Worcestor is famed for it's sauce, and Stratford-upon-Avon is in Warwickshire (thanks to @Going Hamateur for the tip)
Digitless pets and fodder alike
Beware where you crawl for some of us bite.
This is Pisces (Fish)
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)
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".
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.