Can I take .25 mgs of Xanax several hours before 5 mgs of Ambien?

There should not be a problem if both meds were prescribed by the same DR., he/she is aware of the potential moderate interaction between both meds and the Dr. has outweighed that the benefits are greater than the potential risks and would not put you in harms way.
If 2 different Drs. prescribed the benzodiazepine and the sedative/hypnotic I suggest you inform them if you have not done so already.