What are the practical limitations on a column family in Cassandra?
I'm not a fan.  It's about as good an idea as creating a relational table named OrdersOrCustomers with columns defined for both.  The storage-engine penalty is slightly lower in Cassandra because of the sparse-cell storage under the hood, but it's still bad practice.

This bites you later when you want to map/reduce over your data; each task will have to scan over all your data, and filter out the rows that don't match what you're actually interested in (e.g., customers).  And good luck making sense of the statistics that Cassandra tracks per-CF.  ("Is this CF the source of 80% of my application reads because of the order data? Or because of the customer sessions it's combined with? Or the other five data types I threw in?")

If you absolutely positively need tens or hundreds of thousands of CFs?  Even then I'd rather run Cassandra without arena allocation, than mutilate my data model like this.