I have a hard time understanding that - I have two UCNX servers in the lab - a 8.5.1 and a 8.0.3.
 
If I run GET https://10.145.206.125/vmrest/users?rowsPerPage=0 against the 8.5.1, it returns the number of users, as it should. If I run the same against the 8.0.3, I get an HTTP error 400 (it returns
 
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><ErrorDetails><errors><code>DATA_EXCEPTION</code><message>A syntax error has occurred.</message></errors></ErrorDetails>)
 
Looking at the WADL, rowsPerPage is supported on both versions - but on 8.0.3 regardless of what URI I try, appending rowsPerPage=0 always gives me the same error.