goat cheese or feta as substitute for guyere cheese in yogurt bechamel sauce
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.