Is there a way to email photos from the Yosemite photos app, ideally with layout options?
Not directly, as far as I'm aware, but you could make a greetings card, then save as PDF…


Select photos to add - 13 pics appears to be the limit for the automatic layouts.  
File menu > Create card  
Choose flat style  
Select a theme, then Landscape or portrait 
Once selected, 'Options' under the card lets you choose number of photos &amp; layout  
The Options 'inspector' is context-sensitive, so depends on what you select as to which editing options you see - number of photos - colour, crop &amp; zoom etc.


When finished, right click anywhere in the card &amp; 'Save Card as PDF'

Attach to email.

This has the advantage that it doesn't need the HTML that would need to be embedded in the email to achieve this without PDF, so it becomes portable, outside email.

Photos will also save this layout permanently, for if you ever need it again, or until manually deleted.