Hi,
I think you use StartExecution method of TestSet Object. I think that if you'd need to run a subset of TestInstances you have to create a temporary TestSet with only those tests you want to run.
As I think those tests are QTP Test you need also to link them each other in the right order. If so please refeer to my article.
Look to the step 7 for the order and link tests.
Have a nice day.
Ciao,
Massimo.