Can I use my existing Redis for a custom rails cache?

The documentation for the ActiveSupport::Cache::RedisCacheStore states:

Take care to use a dedicated Redis cache rather than pointing this at your existing Redis server. It won't cope well with mixed usage patterns and it won't expire cache entries by default.

Is this advice still true in general, especially when talking about custom data caches, not page (fragment) caches?

Or, more specifically, If I'm building a custom cache for specific "costly" backend calls to a slow 3rd-party API and I set an explicit expires_in value on my cache (or all my cached values), does this advice apply to me at all?