We present a systematic study of the abundancies of metal-doped cluster cations MxCn+ (M=Ca,Sc, Y, La, Ce, Gd; x=1,2) produced in a laser vaporization source. The mass spectra of MCn+ and M2Cn+ reveal the onset of endohedrally doped fullerenes from which the smallest possible cage sizes can be deduced. Y2Cn and La2Cn show a distinct transition from alternation odd-even to even-odd at n=69 and n=71, respectively, which can be explained by a substantial change in the doped fullerene structure, i.e., from networked to bi-endohedral systems. A metal induced cluster formation pathway is briefly discussed.