Creating a game from scratch can take you alot of time, it's like reinventing the wheel.Sure you can make it from scratch depends on you.
Using engines can give you the tools to create the RTS much faster.
I can't recommend a engine for you because these are opinions.
Check out whats suits for you and what works nicer to work with.
But you can basically create an RTS from almost all engines.
i hope this answers your question.