How can I stop sending logging to local /var/log/ltm and send only to splunk via HSL

It does also send the data to splunk. I am concerned about filling up /var/log/ltm file system.

Logrotate will restrict the size of the files in /var/log, but you can still outlog logrotate and cause space issues.

I am using "log local0.info ...." to capture my log traffic - is that why logging is still going to /var/log/ltm ? How do I get it to only send my log data to splunk?

Open the HSL destination and log directly using HSL::send

Second question: I have about 1000 virtual servers that I need to execute my logging collection irule on. Is there a global place that I can just put this irule once instead of adding it to every virtual server ?

No. There is no inheritance mechanism for virtuals, so you need to add your logging irule to each virtual. You could write a bash script to iterate through all the virtuals and modify them, or use iControl.