Tadco is the romantic paring of Marco and Tad. They have met several times. Kelly claims Tad is Marco's best friend and Tad refers to him as "bestie".
Probability of Canon Edit
Although they've formally met they both of them have had minimal interaction. Tad seems to fond of Marco, but Marco seems to be commonly annoyed by Tad's actions. Since Marco has a crush on Star and that Tad still seems to like Kelly the canon probability would be 0%.