"Action and verdict combinations" topic on page 515 in SBG Admin Guide provides a good description of how action combinations work and some of the limitations that you should be aware of.
Here is some explanation specific to your case that will help in understanding and interpreting the tables listed in that topic.
This is how I arrived at the above conclusion:
a) In general, the actions invoked by each policy determine how a message is processed, not the policies themselves. However, there are exceptions to this rule.
Since the actions in your case are not the result of two or more compliance policies, you don't need to worry about the exceptions in this specific case.
b) According to Table A-1:
i) "Modify the subject line" action belongs to Processing Category of "Modify"
ii) "Hold message in Spam Quarantine" action belongs to Processing Category of "Route"
This means you are combining actions from Modify and Route processing categories in this specific case.
c) Some special rules apply to combinations of actions belonging to Route and Modify categories, but those don't apply to the specific action combinations of "Modify the subject line" and "Hold message in Spam Quarantine"
d) According to Table A-2, action combinations of Modify and Route processing categories result in "Modify + Route" action.
So this means that the combined action of "Modify the subject line" and "Hold message in Spam Quarantine" will be taken on the message in your case and you will have the desired effect result of "SPF failure emails having subject appended and end up in the spam quarantine".
Hope this explanation is useful.
Thanks
Adnan