Therapy Dogs are not so much trained for the job but rather enhanced. Dogs can not be trained to enjoy the interaction with people other than their owners.
A Therapy Dog needs obedience training and many are trained a trick or two to entertain but their basic personalities are to be outgoing.
A more aloof dog can be trained to put up with touches etc. from those they do not wish to be around but this is not a safe mode for any length of time. If a dog is stressed they may for awhile follow training but there reaches a point that they will begin to go against training.
Would training him for therapy take away from his home guarding?
The answer to this would first be what you consider his home guarding?
Barking to alert that someone is here? (He could be barking because he is happy to have company, he is barking because he knows doing so pleases you, or he may be barking because he doesn't like non-family members in his area.
At eight months I would say to keep up with obedience work and make sure you socialize him as much as possible - this is with other people and controlled situations around other dogs. Over the next months you will have a better idea of how he will mature.
Honestly, it is not easy to give advise over the Internet. When your dog reacted to another dog was it with true aggression or could it have been a startle reaction?
Simple answer for therapy work vrs. guarding/alert. If you want to lean more to personal protection or a dog that not only alerts but reacts then I would say therapy work would be in conflict to each other and not something that would give good or safe results.
We can not always chose what type of work that is a best fit for our dogs. Sometimes we have to just let them lead us as they develop and show us what they are comfortable with.