I just bought an expand scroll stable and then I wondered (after I used it) if it would affect my building.  To try to go find an answer I went to the list of recipes, looked at the Stable Lv.5 and saw it said "A building where you can keep up to 250 pets." 

The thought struck me that if I used the expand scroll, it would only boost my stable capacity by 40 because now I am at 210 pet slots.  So, should I have used the expand scroll stable after?  Am I out 20 gold?

It doesn't matter if you use the scroll before or after, you'll still get 50 additional slots when you build the lvl 5 stable which will bring your total slots to 260.

Lytala is correct- The expand stable scrolls that you have used are kept track of. Therefore if you have a level 5 stable your base amount of pets is 250. If you've used 5 expand stable scrolls then you can hold 300 pets. That 50 is added after the stable max. :)

Ok good, that worried me for a second because of the wording.  And thanks for the quick response.