  In paired Fermi systems, strong many-body effects exhibit in the crossover
regime between the Bardeen-Cooper-Schrieffer (BCS) and the Bose-Einstein
condensation (BEC) limits. The concept of the BCS-BEC crossover, which is
studied intensively in the research field of cold atoms, has been extended to
condensed matters. Here, by analyzing the typical superconductors within the
BCS-BEC phase diagram, we find that FeSe-based superconductors are prone to
shift their positions in the BCS-BEC crossover regime by charge doping or
substrate substitution, since their Fermi energies and the superconducting gap
sizes are comparable. Especially at the interface of a single-layer FeSe on
SrTiO3 substrate, the superconductivity is relocated closer to the crossover
unitary than other doped FeSe-based materials, indicating that the pairing
interaction is effectively modulated. We further show that hole-doping can
drive the interfacial system into the phase with possible pre-paired electrons,
demonstrating its flexible tunability within the BCS-BEC crossover regime.
