Cyndane is still stronger than Graendal, who is much, much stronger than Moiraine, and stronger than even Nynaeve.

Most people are of the opinion that this sort of thing is way beyond the Finns's abilities anyway (hopefully, TOM and the TOG episode will clarify a lot of those questions). Jordan made a point that their powers are limited and not "god like", and a lot of people are of the opinion that even a godly being like Shai'tan can't really alter someone's ability to channel (ie: he can't give boosts, and he can't reduce the ability for real, that it will turn out Cyndane's strength was reduced almost symbolically by a partial TP shield to punish her for what she did to Asmodean). Otherwise, all he can do is to remove your ability, by shielding or by transmigration into a body that can't channnel.

The Finns actually don't seem to have any "special powers" in matters related to the OP. They didn't make Mat immune to the OP, they rather gave him a ter'angreal that melted weaves, ignoring the part where Mat asked to be rid of Aes Sedai period. They decided interpreting his request to be rid of AS and the OP to mean he wanted to be protected from their weaving was a fair enough bargain (it sounds like the Finns cheat because they really don't want to have to refuse a deal). They didn't give him his memories back either, they didn't have his lost memories... he wasn't precise enough in his request, so they decided giving him a bunch of memories, a lot of them that would be useful to his destiny, could meet the letter, if not the spirit, of what he asked for. A lot of people believe like me the price the Finns ask in return is always a link to you to get to collect what you experience in life (but you can negotiate for how long this link will work, or maybe for which areas of your experiences will not to be collected etc.) - that's the only thing they're interested in - and their "gifts" are most often related to knowledge you wish to have, that their "powers" are pretty much related to mind-reading/memory collecting, not alterations to your person at large. We know they can't change the pattern either.).

IMO, the sort of gifts you can't ask of the Foxes are stuff like:

- I want to be stronger in the OP
- I want to be more beautiful
- I want this or that to happen in my life
- I want this or that Talent

while the sort of stuff you can ask of them is:

- I want to know how to make a device to travel between worlds
- I want to know as much as possible about the last Age
- I want to be able to read as many ancient languages as possible.
- I want an artefact that can make X.
- I want to learn a way to use the Portal Stones
- I want to know as much as possible about weaving saidar

I think after Nynaeve/Flinn went there, you could even ask "I want to learn all you have about the topic of healing severing". If they are truly the first to have done it among everyone who visited the Finns, then they would have no answer for you.

That kind of stuff, there's a very good chance they have in their collection (I think they collect artefacts from all eras and worlds, and they have their own skills/technology to make some too) and they have in the collected memories of everyone who've ever made a deal with them, and they have the ability to imprint them in your mind.

If you asked to learn all there is to learn about the WOS for instance, they could give you the relevant memories from everyone who've visited them who lived during the war, and all those from people who have read books about the WOS, down to innacurate scraps of information Joe Blow from Hawkwing's time knew about the WOS.

Basically, I think the Foxes are mostly scholars/collectors of everything they could get their hands on, memories and objects, from as many Ages back in time as they've been around and in contact with other realms.