The only real issue caused by mixing WinForms and WPF is maintainability.
When trying to add WPF functionality to none-WPF applications, you will need to import several WinForms assemblies, which will in many cases have very similar names to their WPF counterparts.
As a rule, it's a good idea to explicitly call any WinForms classes by their fully qualified name, rather than including a broad using
statement.
The post here (C# trayicon using wpf) is a perfectly acceptable way to achieve what you're aiming to do, and will be the quickest option to get your project finished.
If you wish to work with just WPF, there is a nitify icon component which replicates the functionality of the WinForms notify icon - http://www.hardcodet.net/wpf-notifyicon
It's not best practice to mix approaches in this way, however in the real world when you need to get the job done, it's sometimes the most effective solution.