In the Minimized view of your Familiars that appear on the main screen, place a small number in the corner of each icon. This allows quick access to levels and which familiars can then be used to level up. Hovering over 15 icons is slow paced and detracts from immersion.
/Edit: One other thing I would like to add is that there is no % shown of XP. Only in the full view can this information be seen. Perhaps a secondary XP bar on the icon under Stamina, or when a Familiar reaches 100% for it's level ( level 21: 100% ) a similar burst of light and notification on screen and in the chat window, could be shown like when a familiar reaches another level.
/Edit: One other thing I would like to add is that there is no % shown of XP. Only in the full view can this information be seen. Perhaps a secondary XP bar on the icon under Stamina, or when a Familiar reaches 100% for it's level ( level 21: 100% ) a similar burst of light and notification on screen and in the chat window, could be shown like when a familiar reaches another level.