Following your logic, it would make more sense that Vegapunk used Paramecia, as those are the ones truly more common (not Zoan). Again, the technology would makes sense to exist due to the Zoan's ability to give a certain consciousness to it's eater. He might have studied how it happens with sentient beings, how they can loose themselves in their devil fruit (Impel Down's minotaur guards), and figured out how to apply it to non-sentient beings. That's what makes sense. I don't understand how smiley creates fruits, but since, once, again, it only creates Zoan-type, it would make more sense that both are related, and not that one happens because of the other.It was because dr Vegapunk created the technology to do it. By default there is nothing that makes it possible for an inanimate object to eat DF. Outside that there hasn't been said anything in the manga about the requirements or limits for an inanimate object to eat a DF. That's it. It's true that the only fruits who have done it so far are zoan types, however the chance that Vegapunk would have gotten his hands on a logia-type and was even allowed to experiment on it, are extremely low considering its value.
So currently there is nothing that underlines that the first premise would be necessarily incomplete. Unless the mange clearly states it only works for zoan types, it's perfectly possible as it was said that he created the technology for objects to eat DF, not zoan DF.
As it stands the only real obstacle in theory for the possibility of Shuusui eating the Mera Mera no Mi, is getting the hands on the technology to do so and we know Doflamingo has a zoan type DF factory on Dressrosa which originates from Vegapunk's research and I doubt Doflamingo/CC didn't at least try other DF types.
It's true that it seems most research has been done towards Zoan types, but that can easily be covered by just it being the most common type of DF and overall the most safe one. In the end there are still only 4 objects that ate a DF, so it's really not enough to rule out the possibility of objects eating other fruits too. Especially not when I think if Shuusui eats the DF, it's going to be intentional, but more of an accidental fluke.
And you said it yourself, it's unlikely they didn't try on other fruits. I say it's because of the consciousness stuff. Likewise, the fact is, it hasn't happened. So either they failed, didn't try or didn't reveal. Then again, their pride possession was a Dragon Smiley, not a logia or a paramecia.