Can you use varm for hot (spicey) food?
We use "sterk" (strong) for that.
Can you also say krydret or is that used for foods with a lot of spices in general, such as cumin or cardamom?
That something is "krydret" only means that there has been added spice(s) to it. It doesn't speak to the degree of spiciness on its own.
You could say "Maten er sterkt krydret" to convey that meaning.
I've had this twice now, one was "the food is warm" & the other was "the food is hot" so how do you know which they're talking about?
In this context, it could be either.
The literal translation of "hot" is "het/heit", so we allow that here, but generally Norwegians are more likely to use "varm" when referring to food - even when you'd call it "hot" in English.
Would you use het more for food that is scalding-hot, and varm for food at normal eating temperature?
For a little wicked diversion, "The Food is Hot" reminds me of the Death Star Canteen sketch by Eddie Izzard. https://www.youtube.com/watch?v=Sv5iEK-IEzw (not expurgated, I'm afraid).
And for neuter it would be something like, "huset er varmt." Right?
Would Matene er varme work?