You Do Need power plug adapter while traveling from French Guiana to Guinea.

Here is Why You Need An Adapter

The outlet and plug type used in Guinea are C, F and K and in French Guiana are C, D and E. This is how they look like.

French Guiana

In French Guiana, outlets of type C, D and E are in use.

Type C

Type D

Type E

Guinea

In Guinea, outlets of type C, F and K are in use.

Type C

Type F

Type K