No one intercedes for those in heaven. Those in heaven can intercede for others, though. In Romans, the Spirit is interceding for those who are not in heaven yet. Jesus desires that the Kingdom of God is made present to all men of all time. Thus, He continues to intercede for them, not those who already have the Kingdom fully within them. A person doesn't have to be a cannonized saint to be in heaven. Cannonized saints are "officially" recognized for the purposes of the Liturgy, but we are encouraged to ask the prayers of any saint whom we believe has been saved (and is in Purgatory or Heaven).
Regards
Well, then who are "saints" to Catholics? We say any saved person. You said that saints can be in purgatory, which leads me to think that Catholic saints are more than "the famous ones". Also, do you disagree that the passage in Romans 8 refers only to living people on earth? The context seems clear to me.
Rom. 8:26-27 : "26 In the same way, the Spirit helps us in our weakness. We do not know what we ought to pray for, but the Spirit himself intercedes for us with groans that words cannot express. 27 And he who searches our hearts knows the mind of the Spirit, because the Spirit intercedes for the saints in accordance with God's will."