I have a mousakka recipe that uses a bechamel made with yogurt and olive oil (instead of heavy cream and lots of butter)that calls for guyere cheese. Can I use feta or goat cheese instead in a becahmel for a mousakka?
Yes, you could use either. A soft goat cheese can be mashed and mixed easily into your yogurt/olive bechamel sauce.
If you use feta, I would suggest that you melt it first. Crumble the feta into small pieces, add a bit of milk, and heat over a double boiler (or bowl set over pot of water), stirring, until it softens and becomes sauce-like.
In both cases, the substitutes may affect the consistency of the bechamel, and you may need to adjust your thickening agent.