Scrap's heart is tied to the type of greeting you chose to say that is human's usal way of greeting people, if you said that humans normally kiss then when Scrap meets Tao and Tao debunks your lie Scrap gets embarresed and you don't gain the heart(the food option only raises his likes and that's it).
To get Tao to meet Scrap take Tao into the junkyard bring him to Scrap's hut, Scrap will stay hidden during the time and you will get some dialogue with Tao, come back without Tao and talk to Scrap about a friend wanting to meet him after that come by with Tao and you'll get them to meet