It doesn’t happen often that an American colossus like Viacom sets its sights on an Italian animation studio — and then, rather than snapping it up outright, agrees to purchase just a minority stake.

But, then again, as Iginio Straffi, 45, the former illustrator and comicbook writer who founded Rainbow Group in 1995, puts it, “there aren’t that many properties out there with figures comparable to the Winx.”

Created by Straffi, these six trendy MTV-generation fairies, which have become one of Europe’s top TV exports, have bewitched tween girls in more than 130 countries, and generated more than $2.7 billion, thanks in part to the theatrical film “Winx 3D” as well as myriad merchandising deals.

The partnership, announced in February, under which Viacom will take a 30% stake in Rainbow, follows a global content agreement inked in September under which Nickelodeon secured TV and merchandising rights to Winx Club in the U.S. and pays TV rights for a large swathe of the planet, including Latin America, Canada and the U.K.

The partnership now expands to the co-development and co-production by Rainbow and Nickelodeon of seasons five and six of “Winx Club.” And, significantly, it will open up the U.S. theatrical market to Rainbow’s upcoming big-budget 3D feature, a gladiator toon penned by Michael J. Wilson (“Ice Age,” “Shark Tale”) with the working title “Versus Roma,” to be completed in a year. The pic figures to go out Stateside via Paramount.

Viacom’s first indication that it had found an important partner in Rainbow might well have been that as soon as the initial deal was signed, it set off a flurry of auctions for merchandising rights to Winx toys, clothes and books, especially in the U.S.

Rainbow has always made a major effort to create properties that lend themselves to merchandising “because we had to, in order to survive and grow,” Straffi says.

And Straffi sees merchandising as more than just the revenue stream — one that accounts for about two-thirds of his business. “If the child reads a book or a comicstrip, or plays videogame, all based on our characters, his interest level will remain high even during the gaps when they are not showing on TV,” he says. “It helps the product’s longevity.”

Straffi is excited about the film side of the alliance, which gives him high hopes that his gladiator spoof, with a budget of more than $40 million, will click with U.S. auds.

This, he says, would mark “the first Italian movie to go out wide in the U.S. since (Roberto Benigni’s) ‘Life Is Beautiful.’?” But unlike “Life,” Straffi’s pic will be in English.

“I can only wish that if it’s successful, Rainbow will become a constant content provider to Nick and other Viacom companies, like Pixar is for Disney.”

But, unlike Pixar, don’t expect Straffi to sell outright anytime soon, though a flotation is “possible in a few years with me still at the helm,” he says.

Straffi is also looking forward to the May 25 opening of his Rome theme park, Magic Land, comprising attractions based on Rainbow’s properties such as “Pop Pixie,” “Monster Allergy” and, of course, the Winx.