1.Maybe some of them know a lot about the ancient weapons and history?
That is possible, as there are some really big names already that we know of we can only imagine. The thing is most of them got away with Strawhat and Co. The ones that weren't in the Marineford War are the ones that had to get out by their own power, do you agree? So that is scary seeing where the Impelt Down actually is
2. How strong are they? Close to Admiral-Level?
We had Shiki, Portgas D. Ace , Jinbe , Crocodile , Shiliew, Catarina Devon, Sanjuan Wolf, Vasco Shot, Avalo Pizarro, Patrick Redfield, Catarina Devon, Sanjuan Wolf, Vasco Shot , Avalo Pizarro.
So seeing that some of them are weaker and monsters like Shiki and Jinbei are more like Admiral-ish power. So, yes could be. 6th level is the place for the biggest monsters...
3. What purpose do they have in the future? Are they gonna be a big power next to Marine/Shichibukai/Yonko/Revolutionaries?
I think they could be some of the road blocks for the Strawhats in side-stories.
The shock Sengoku had when he heard of the break-outs shows how fearful/strong those guys must be. I wonder what Oda has in store, what do you think?
Cool fruit powers, interesting personas and cool backstories like Shiki had. How they were inprisond by the Marines and what was the reason.