While there is not really a maximum number of inlets that can be installed, to maintain the best performance of your system, keep the number of inlets to a minimum. By increasing the number of inlet, you may need to increase the size of the power unit.
YES. While installation into an existing home will present it's own set of challenges, it can be accomplished. Knowing the layout of your home and planning the installation completely before starting will help ease the process. Also look for alternative ways for running the systems piping, such as through closest or other hidden chambers.