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

Here is Why You Need An Adapter

The outlet and plug type used in Guinea are C, F and K and in Tonga is I. This is how they look like.

Tonga

In Tonga, outlets of type I are in use.

Type I

Guinea

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

Type C

Type F

Type K