Can Festivizers be applied to Specialized Killstreak weapons without removing the killstreak?

Solution 1:

In general:

Festivizers, name tags, description tags, filters, paint, strange parts, and killstreak kits (all 3 kinds) are all independent of each other. They can all be applied and removed independently of each other. The only exception is a strangifier, which when applied is permanent.

Solution 2:

I received my first festivizer recently and wanted to use it to festivize a weapon. To my surprise, only a few weapons could be used with the fesivizer.

Here's a link a later found with all the items you can use it with. Sadly, i can't just use it with anything. https://wiki.teamfortress.com/wiki/Template:Festivizer

For the weapon you were planning to use it with, make sure you CAN festivize it before buying it.