No, you should not install NSI 1.5.1 after updating to System 7.5.3 with System 7.5 Update 2.0. In the System 7.5 Update 2.0 Read Me it states:
"Files no longer needed
=======================
The Update automatically deletes the files listed below -- unless you renamed any of them, in which case, you should remove the files.
** Several standalone files are now merged into the System 7.5 Update enabler or the System file. This update deletes the old (standalone) versions of the following files:
-...
- Network Software Installer (v1.5.1 and later)"
NSI 1.5.1 should be used with System 7.0 to System 7.5.1.