I can't speak for a current safe limit, but it isn't very hard to transfer funds between various cryptocurrencies. Tumbling is not secure with a large amount of coins. All you have to do is monitor all transactions and figure out what went in and what came back out. If you start splitting things off in small amounts on various blockchains things become much harder to track.

This is assuming the attackers know what they are doing. I would be against that.