Hi sweetie, it's not really about mixing foods it's more about not using foods for their purpose. For example if you use potato to make crisps to snack on, your supposed to syn them because your changing its purpose, and the same with grinding down couscous to make flour etc. in this case your using smash like flour so your supposed to syn it.
I use smash as a bread crumb on chicken and I Dnt syn it, because I think to me it's not worth it, I don't do it a lot and I Dnt use all my syns so having a little bit of smash won't kill me but technically it should have a few syns. Hope that helped. Ask again if it didn't and ill try explain a different way x