Using modules with almost similar functionality together on production site I have 2 modules with similar/alternative available and I have to use both. Those are Entity Reference and Search API.

Now the situation is that there are few modules which require References and Search modules as a dependency. It is a need and I have to use it.

Can I use all these modules at the same time on a production site?

Is there any possibility of clash or site crash?
Yes, there's always a possibility that modules will be incompatible with other modules that do the same thing. 

There's also the possibility that modules whose functionality is completely unrelated will be incompatible with one another. Such is the nature of a modularly extensible system.

The only way you'll know is if:


The documentation for one or more modules list their known issues with other modules, or
You thoroughly test the two modules while both installed on a single site