I see no benefit to python if I know java. All the spark libraries do everything I need, particularly with the spark 2 apis. To me, python is for folks who want to crunch something together without knowing a lot of syntax pomp and circumstance. But that's just my religion.
Is there any detriment to mixing some java classes with scala in an app?
When you do things right, people won't be sure you've done anything at all.
No, but you in most cases you wouldn't want to unless something needs a java type specifically. For example, lists , longs, dates, some times you need java. In a lot of cases you can add .toJava or toScala to convert from one to another.
I'd just chip in and say that although there is interop between Scala and Java, it's generally not very straight forward. Specifically, if you convert from a Java collection to Scala or the other way round, there will be a lot of noise/boilerplate and if you don't bother, you won't be taking advantage of the language features (for example, you'll end up calling flatmap and not flatMap when using Java collections in Scala and won't be able to use for-comprehension).