please find the answers to your questions below.
1. Whether McAfee and Symantec can coexist?
No, even if its possible it is never recommended to install two different AV on same machines, because it causes performance degrading and there is a high possibility that one AV may flag other AV's signature as threat and causing you more trouble than the security it has to offer so never install two AV in a machine.
2. Can Symantec endpoint installation script automatically uninstall McAfee agent on the end points.
Yes, Symantec has built in a feature while you build the install packages. you need to explicitly enable this under the install settings.
for more information please refer to the write up by Mithun
3. What is the recommended migration approach.
since you are migrating from different AV, you can start off directly by installing the latest version which is SEPM 12.1 RU6 MP1a without worrying much about the compatibility. this version is also compatible with the latest Windows 10 O/S as well. let me know if you have more questions.