Beside Xian Grand Mosque & Muslim Quarter & Muslim street are there any Islamic center/istitution?

Hey, near the Grand Mosque, there is another mosque located in North Guangji Street. You can walk to the west from Grand Mosque, when you reach North Guangji Street, just turn to south, and you’ll see it.

To have a special Muslim themed tour, I suggest you turn to a travel agency, who would arrange you many surprises.

if you don't mind kindly give me the agency name for my reference.

I know a good travel agency headquartered in XA arranges Muslim trips - Travel China Guide. They will offer you superb service. If you are interested, here is their contact:
traveltravelchinaguide.com