In this work, the small signal stability of a two-area four-machine test system and a reduced CIGRE Nordic system is analyzed. The effect on the stability of the system at high penetration levels of non-synchronous generation (NSG) as well as the placement and distribution of NSG are discussed in detail. The focus of this paper is on the behavior of the frequency and damping of the inter-area modes as the system approaches very low levels of inertia. A methodology to identify the inter-area mode frequency based on the system inertia is proposed. The developed theory is compared to the obtained eigen-value results for the small and large test systems, it is shown that the proposed methodology successfully predicts the inter-area mode frequency of the two test bed systems. Time domain simulations of the smaller test system are performed as a point of reference, in order to validate the eigen-value analysis. Additionally, the paper explores the impact of the inertia of grid-forming converters on the behavior of the inter-area mode frequency. Although, the focus of this paper is on the inter-area oscillations of two-area systems; the proposed theory is also applied to a multi-area frequency mode for the large test system and is validated.