Yes, you can pull only the last system modstamp.
First, are you connecting to the Task system modstamp field through the use of a Child Relationship (see this tutorial for a reference: http://help.skuidify.com/m/models-conditions-filters/l/102520?data-resolve-url=true&data-manual-…)?
If so, you can limit the system modstamp to one Task on an Opportunity by going back to the Skuid Page Buillder. Click on the Models tab. Click on the Tasks child field. Select the Properties section. Change Max # of Records to 1.
This limits the number of Tasks to 1. From here, you can also set a condition on this field if you feel that a condition limiting the field would be more helpful.
Let us know if this helps! If not, then we will troubleshoot further.
-Josh
