You absolutely need to have pre op bloodwork done, especially with the high incidence of liver disease in Yorkies. Bloodwork will alert your vet to any hidden health problems that might make it hard for Chloe to process anesthesia.
Please read this thread about little Toby and his near fatal reaction to anesthesia during his neuter:
http://www.yorkietalk.com/forums/sic...on-update.html
Does Chloe have any retained baby teeth? Your vet will normally remove them during the spay, but the ASPCA vet will not. If you have to have it done separately, it will end up costing you more in the long run.