This study aims to develop a novel pinhole collimator system for high resolution and detection efficiency imaging of mice, rats and rabbits using a 3-head SPECT camera (Philips IRIX). Utilizing the entire camera by a single pinhole (SPH) collimator with a larger collimator length (CL) and magnification offers higher spatial resolution, while using a multi-pinhole (MPH) collimator increases the detection efficiency, provided that a shorter CL with less magnification is used with minimum multiplexing and loss of resolution. Optimum imaging of different sized animals also requires adjustable CL. The proposed collimator system consists of a base of 14cm and two cones with heights of 3.5cm and 11cm. They can be assembled into a SPH collimator with a CL of 25cm, or two MPH collimators with CLs of 17.5cm and 14cm that contain changeable pinhole patterns. Resolution and detection efficiency of this system were computed using theoretical predictions and Monte Carlo simulations. The determined FWHMs of the point spread functions for imaging mice, rats and rabbits were 1.14mm, 1.27mm and 1.69mm, respectively for a SPH collimator with 1mm aperture size. The detection efficiency ratio of a 7-pinhole collimator with CL of 17.5cm and the SPH collimator was 6:1 at a point-to-aperture distance of 3.8cm. Our results demonstrate that the proposed collimator system allows flexible SPH and MPH SPECT imaging of different sized small animals while achieving high resolution and detection efficiency.