Hi Ken,
There isn't any logic in default LTM config to stop marking pool members down if they fail the monitor checks. You could potentially build this into an external monitor script using a template:
http://devcentral.f5.com/wiki/default.aspx/AdvDesignConfig/TemplateForExternalLtmMonitors.html
If you want to get a count of the total members of a pool you can use the members command. To get a count of the active members you can use active_members:
http://devcentral.f5.com/wiki/iRules.members.ashx
http://devcentral.f5.com/wiki/iRules.active_members.ashx
You could check the ratio of active members / total members to determine if you want to use a second pool.
Aaron