Understanding parameter types during multidimensional array creation

Does the expression >: Char with Int <: AnyVal mean - Any type (_1) that is a supertype of Char with Int but a subtype of AnyVal?

Yes, this is correct.

are we defining a new Char class with the 'traits' of Int

We are not defining anything. Are you define a 'class' when you write 'x: Int => String' ? No, it's just type expression. This is a syntax that allows you to express a type. With keyword 'with' you construct a type that is a subtype of Char and subtype of Int. This expression can be used as union type encoding (https://stackoverflow.com/a/6883076/14044371). A union type is a type that combines few types in a single type with logical "Or". It also called "type disjunction". Intuitively you can try to understand this by imagining examples:

// what is the possible supertypes for type 'Char with Int'?
// it is 'Char with Int',  Int, Char, AnyVal, Any 
val x: Char with Int = Char with Int
val a: Int = Char with Int
val b: Char = Char with Int
val c: AnyVal = Char with Int
val d: An y= Char with Int

so T >: Char with Int and T <: AnyVal almost equivalent to Char Or Int

For intuition on Char Or Int I can give you this:

val a: Char Or Int = 1 // is it correct? yes it is
val b: Char Or Int = 'a' // it is correct too
// after that, you can pattern match on such type in a safe manner
def foo(x: Char Or Int) match {
  case Int => ...
  case Char => ...
}

So.

You are correct in your second question.

You are not quite right in the first, because it is not 'class' or 'trait' - it is type expression. This is not intuitive, because you have the same word 'with' like in inheritance of classes and traits. But this word means a little bit different things in different contexts.

This construction is the way to express union type that suits well as a type for the original expression val tmix1 = Array(Array[Int](1,2,3), Array[Char]('b','f'))