Haiving a schtick active (and in use) disallows the use of other forms of attack, so you can mix Lightsabre and Combat schticks but if you try and use them at the same time you won't be attacking that turn.
The main use of mixing them is to quickly shift between attack modes, voluntarily not using a scthick you have in as you wish. So if you have, say, Block and Eye of the Storm, it could be on the idea that when attacking with your lightsabre you use Block, when using your guns you use Eye of the Storm. And so on.
(Why not just switch in the one as appropriate? Well, your third slot might already be doing that, switching as needed between, say, Forward Guard and Carnival of Carnage).
If you want to keep things simple, just use one type at a time though. If you are feeling fancy and like to stay flexible, the option is there to mix types so long as you do not attempt tp use two different types of schtick in the same turn.
Looking at Rianna's choice, she should change that.