yeah, these are all just speculations based on the whole cloaked thing and magical abilities and just dubbed "potential candidates". the "horn playing druid" isn't his real name, just a description based on appearance that fans have used. he could very well be the same type of cloaked being that was found in Simon's Quest.
Shaft, Bartleys, Rohan Krause, Necromancer are all just speculations as well. I just noticed that Isaac and his companions (who he killed eventually) were at Dracula's Castle and wore cloaks like that. I wonder if there were more such people at Dracula's Castle and if the movement to resurrect him stems from these followers of him in the CoD manga. Perhaps they started out as Gypsies, but eventually their cult spread and they accepted "normal" humans with lusts for power, immortality, and evil in their hearts, so that people like Shaft might not actually have Gypsy genetic roots, but their movement did.
The sprite used for the Ferryman in Simon's Quest was the same as the merchants and graveyard spirits. I've always assumed they were unrelated because of the limited amount of sprites they created for the game, but what if the sprite is tying together an entire culture? There's a clue that says the Ferryman loves garlic, but I wonder if that's just a translation issue and the clue might have been intended to refer to the graveyard spirit. The origins of things in Castlevania may also be different than historical and mythological origins.
And of course Belnades being gypsies is also speculation, but I think it nicely ties things together. In the end, it's all speculation. According to the Japanese, Sypha was a witch, but she was essentially raised by the church, who took pity on her, after her family had been killed. The Belnades, including Charlotte and Yoko, stems from a long tradition of siding with the church, and is a bit of a case of strange bed fellows. Siding with the church or Dracula seems to be a decision based on who an individual gypsy holds responsible for the humans/church's persecution of their people.