Sure, Kribn's can live with guppies, however I'm not sure how long the Guppies will live with the Krib's, especially once they pair off. As for what size for them to pair, I recomend mking sure that your male is about double the size of the female. In the wild the younger females are generally attracted to the older more mature males. When I had a male and female the same size, the female beat the male up pretty good. Once I had a much arger male and a smaller female, they paired within a week. Read around here and you will find many others have had the same experience with the smaller female and the larger male.
Do you have room for the reprocussions if they decide to breed?
Colin