I get the sense that Either Frog Sage Mode is fundamentaly different from the Snake/Dragon Sage mode, or Kabuto was only able to pull it off because he had stolen Juugo's cells and ability.
As for Tsunade, I suspect that she may have had the neccessary chakra levels for frog Sage mode. Tsunade is really a chakra beast (IMO), Also consider that Tsunade is Hashirama's and Mito's grand daughter. She certainly has some impressive Senju/Uzumaki pedigree.
Hope kishi gives us more info as to Who is related to Whom. I'd like to know if Minato is somehow connected to the Senju/Tsunade, or to whomever.