Mediation is a relative term. That is you can't mediate something from yourself to someone else per se. For example, Christ mediates between God and man (being Himself both).
Thus, any of us can mediate (go between or relay something/be the instrument of confering something to someone else):
1 Timothy 2:2
First of all, then, I urge that petitions, prayers, intercessions,
and thanksgiving be offered on behalf of all men for kings and all
those in authority, so that we may lead tranquil and quiet lives in
all godliness and dignity.
Mary is said to be Mediatrix of all graces in a relative sense, since she is only said to mediate from Christ to the rest of us.
[That Mary is Mediatrix of all graces is not as yet Catholic dogma; and it is very much debated among Catholics—you may believe or not believe Mary is Mediatrix of all graces, although it would be prudent, seeing the consistency with which it has been taught, to consider it something worthy of belief—it has not been definitively settled by the Magisterium/teaching authority of the Church. This quasi dogma is closely linked to Mary's being Co-Redemptrix (see below), and so many who reject this her being Co-Mediatrix with Christ also reject the former.]
For example, Pope Leo XIII said:
...it be also affirmed that, by the will of God, Mary is the intermediary
through whom is distributed unto us this immense treasure of mercies
gathered by God, for mercy and truth were created by Jesus Christ.
Thus as no man goes to the Father but by the Son, so no man goes to
Christ but by his Mother. §
§ Leo XIII, Encyclical on the Rosary Octobri mense, September 21, 1891
But Blessed Pope Pius IX implies Mary is not exclusively the only Mediatrix, only that she is the ultimate, you could say, "among women":
...with her only-begotten Son,* is the most powerful Mediatrix and
Conciliatrix in the whole world †
† Pius IX, Apostolic Constitution Ineffabilis Deus, December 8, 1854. [the same papal document defining the Immaculate Conception of Mary]
- [note mine—1 Timothy 2:5]
This is powerful stuff. Note that he is speaking objectively. Like how Abraham didn't know he was actually going to the Father but through the Son. You mustn't strictly be aware that God has thus arranged the economy of His grace, but accept it once known.
She doesn't breach or usurp Christ's ultimate Mediatorship a) because 'usurp' implies a certain violence or a hesistance or reluctance or unwillingness to share your perogative, which we do not know Christ does with Mary, and expect that for His blessed mother He loves her much indeed b) that she is Mediatrix does not imply she doesn't need the grace from God & c) nowhere is it implied she can 'go past' Christ to get grace from God the Father: thus she is only an instrument of Christ (in a more loving way than that sounds).
Mary is an aqueduct which God has chosen, freely, to channel His graces through. Simply out of love, and in His eternal wisdom.
"Is it possible to say the saints are "Mediators of certain graces", according to Catholicism?"
In every way, yes. But they are not the exclusive mediators of any grace. Any more than Christ cannot chose to grant graces to anyone He wishes in an otherwise fashion than through His beloved mother, which we hold with a pious faith that He is not wont to do.
But yes, because if this is the case that God gives all His graces through Mary, then the saints may be the special mediators for certain graces, but only to a relative degree: no saint has a monopoly over one kind of God's graces.
Mary as Co-Redemptrix
This is closely linked and is very much analagous to the other doctrine of Mary's being Mediatrix with Christ, of all graces.
The core underlying link is that Mary is the Eve to Christ the New Adam:
Adam utlimately was responsible for the Fall.
Christ, called in Scripture "the Last Adam," was utlimately responsible for the Redemption.
Adam renames the Woman "Eve" ('Life') because "she was to be the mother of all the living," Christ, who exclusively called His mother "Woman," and named her "mother of all the living" in Christ: "behold thy mother" (Rev 12:1,5,17).
But Eve did play a part in the Fall. She "instigated" it or in any event without her it might not have played out the same way. Thus Mary is viewed as a major part of God's plan in the Redmption. Mary "encourages" or "instigates" Christ's ministry and ultimate Redemption of mankind in asking Him to 'start the timer,' as it were, on His Passion and death by working His first miracle at Cana: Eve began the work of the first Adam, Mary began the work of the "Last Adam," Jesus Christ.
Scripture records that Mary also suffered by way of "sword" through her soul. Which doesn't sound very nice: that's because being hoyl and knowing that her Son was divinely holy, and innocent, nonetheless watched as His precious blood dripped down His unrecognisable body. And we believe that her sufferings God put into a kind of 'treasury', as all sufferings of good people do, wherefrom God can bestow them justly on others who said saints are willing that they should be bestowed.
Thus Mary's sufferings,though absolutely inferior and categorically separate from that of Jesus', may be, in a real way, the difference between some people getting graces for her sake than if she had not suffered. So she contributes for certain sinners a mini passion. Not to be diminished in reality, but, out of protection of the uniqueness and absolute superiority of Jesus' Passion, we ought to be very careful in speaking about.
Thus, they are kind of 'in it together' at the Garden, Adam and Eve. And there is a certain continuity with Christ and Mary, except Mary is not God or divine in any way, but she wants only what Christ wants and so to ask of Mary is to ask Christ. Because He won't refuse her anything nor do anything for her which would breach His justice. yet He is happier and more glad to fulfill the request of His mother as aparticularly exemplary "spiri[t] of the righteous made perfect".
Thus she is the greatest advocate for us with Christ for graces, and thus with God.