Fe+2 (also Co+2, Ni+2, Cu+2, Zn+2, and Sn+2) all have the same hydrated diameter as Ca+2 = 600pm. Given their same charge and hydrated diameter, I believe they could all go in through calcium channels.
This also shows why potassium is desired over Na, 300pm and 450pm respectively. And when the protoplasm structure changes Na will substitute for K (100pm vs 160pm unhydrated diameters). Dr Ling discusses this in terms of c-values. I’ll make a separate post about that later.
Kathleen Stewart 