Can i replace part of pool pump piping with smaller pipe gluing pipe inside instead of coupler and same size pipe shouldnt be much smaller?

Yes, you can replace part of the pool pump piping with a smaller pipe, but it's important to ensure that the reduction in diameter doesn't significantly restrict water flow or create pressure issues. Using glue to secure the smaller pipe inside the existing pipe is possible, but make sure the connections are watertight and stable. Ideally, it's better to use couplers to maintain a proper flow and reduce potential stress on the system. Always check the manufacturer's specifications for your pump to avoid any performance issues.