Any tcp port number (1 to 65535) can be used for a probe. Did you create the probe following the syntax shown in the ACE documentation? If you do, you should end up with a script looking something like this:
probe tcp TCP_42511_PROBE
port 42511
interval 5
faildetect 2
passdetect interval 30
Once you have that in your script, you can apply the defined probe to the rserver(s) defined in a serverfarm script section. Something like:
serverfarm host [your_host_name]
description [serverfarm_desc]
predictor leastconns
rserver [serverfarm_rserver_1]
probe TCP_42511_PROBE
inservice
rserver [serverfarm_rserver_2, etc]
probe TCP_42511_PROBE
inservice
Of course your interval parameters, predictor method etc. may vary.
In general, you will get better responses for ACE-related questions in the Application Networking forum in the Data Center section of CSC.
Hope this helps.