Is it permissible to put Soup Mandels in hot (Yad Soledes) soup on Shabbos, assuming the soup bowl was filled in a standard way - pot taken off of fire, soup ladle inserted into pot and soup removed and put into the bowl in which it is served.
Yes. You can even put it in a cli rishon that's off the fire.
שקדיות מרק ... הנמכרות כשהן מוכנות לאכילה, מותר לתת אפילו בכלי ראשון, אם אינו עומד על גבי האש...
Soup mandels ... that are sold ready to eat, it is permissible to place them even in a cli rishon, if it is not on the fire. (Translation mine)
Source: Shmirat Shabbat C'Hilchata, 1:61
When the pot is taken off the fire, it is still a kli rishon. The soup ladel is a kli sheni and the bowl a kli shlishi.
As such, it depends on what you hold and how machmir you are on Shabbos. Either way you have who to rely on. IIUC, most people are lenient.