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Another from at least thirty years (or longer) ago.
Set a few years after Earth's defeat and the alien overlords are involved in various power grabs and empire building.

They have been supreme slave masters around the galaxy for countless generations and have evolved until no limbs (everything done by body servants) and therefore use only their tongues for pointing.

I want to say the next slave race down in the hierarchy is semi-evolved to a similar condition as they also have body servers from yet another slave species, quite a few species are in the book.

One of these overlords gets involved with a crime syndicate and is tricked into believing unpainted plastic model planes have great commercial value to humans. He hoards these in glee until he tries to pay off another criminal gang "Why we want that shit?" and realises he's been tricked.

Another alien likes rolling around on human skin rugs and is ever alert for hairy bodied men as it likes the tickle sensation.

I think (!) There is some kind of alien wipe out towards the end (like in Battlefield Earth) and the freed species around the galaxy look to mankind, however I'm not certain on this.

It was a medium size hardback book

This is a bit of a longshot, but could you be thinking of I, Weapon by Charles Runyon?

Ancestry: Programmed

Destiny: Unpredictable

To create the man named Raki, Earth had broken its most rigid taboos against interbreeding.

His grandparents were a four-foot tall female genius; a rapacious, blue-skinned space brigand; an enormous superstud gladiator with long, silky fur; and a blind telepathic huntress.

His parents were the most gifted creatures ever to walk the earth—until Raki was born to surpass even them.

But the supreme computer that had calculated the mating necessary to produce Raki refused to predict his chances in his confrontation with the alien Vim in the very heart of their eternally expanding empire…

…for not even the Vim themselves knew the secret of their power and of all the universe’s peril…

It has the humans conquered by aliens, and thereby forced into a massive division in phenotypes, ranging from aquatic humans to bird-like ones to the Ungul who have been raised to be harvested for meat and skin to the last "normal" humans who have bioengineered themselves to survive in an underground base on the moon. I don't remember the aliens specifically pointing with their tongues, but it's a detail which fits the setting.

The Ungul are watched over by another sort of humans, the Grithies, who have hugely elongated earlobes because the aliens who farm Earth like the look of earlobes, and bred for the quality.

Sadly, the model planes are also a miss in terms of details, but I figured that, even if it's not the answer, a partial answer can also be useful.

Childhood's End by Arthur C. Clarke (1987)

The Overlords appeared suddenly over every city--intellectually, technologically, and militarily superior to humankind. Benevolent, they made few demands: unify earth, eliminate poverty, and end war. With little rebellion, humankind agreed, and a golden age began.

But at what cost? With the advent of peace, man ceases to strive for creative greatness, and a malaise settles over the human race. To those who resist, it becomes evident that the Overlords have an agenda of their own. As civilization approaches the crossroads, will the Overlords spell the end for humankind . . . or the beginning?