What species might it be? And is it full grown? :)
Watch your tiger barbs. You probably have heard that they will chase and sometimes nip at other fish. Poor companions anyway for guppies, they often behave better in schools of 6-10 because they are busy interacting with each other. Its is almost like they are so busy watching their neighbors and guarding their flanks, that they don't go after someone else. :)
If they are accustomed to the mollie and shrimp, that may not be an issue. Watch to see if one badgers the other though. The mollie looks like a female, they will be "fond" of baby mollies.
All the best!
u.s.