What should one do when one sees this (an enemy fleet from planet Kanopos has been sighted near your planet)?

Be prepared for a possible attack.

As in, how?

Go to the Resource menu and set all of your resources to 0% production.

Spend all of the resources that you can't fleet save.

Deploy a fleet save with your ships and make sure you max out their cargo holds.

Basically it means somebody has just used an espionage ship on you, if it worked they will know how many resources you have. They could also know your fleet size and composition, your defensive structures, your buildings and your research. If they see something tasty they will probably launch and attack fleet immediatley after.

-Istalris-

Are we allowed to have small shield and large shield at the same time? or do i have to destroy the small one to build the large one?

You can have both at the same time.