I understand alarplasty is usually done alongside septo/rhinoplasty.

Hello! If alarplasty is performed it is typically and ideally done at the same time as rhinoplasty. Good luck!

Hello and thank you for your question.  Alarplasty is typically performed at the same time as rhinoplasty in patients who need this procedure.  If it is not done at the same time for some reason, then I would recommend waiting six months.
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon

There are no rules that require unnecessary surgeries. It surgery is done for its own merits and needs. If you only need alarplasty, you can have it without any other surgery to your nose. 

An alar plasty can be performed as part of a rhinoplasty procedure, or as a stand-alone procedure  when patients only wish to have the nostrils narrowed.  A septoplasty is performed to improve airflow in the back of the nose, and is usually performed for medical necessity.  A septoplasty will not change the shape of the nose.   For more information about the differences  between a septoplasty and rhinoplasty and many before and after examples, please see the link and the video below

Typically they are indeed done together. If someone decides do alter the alae at a later date, I would want all tissues/scarring to be soft and stable before doing this. At a minimum, I would suggest six months and possibly longer.

In cases where the patient or surgeon elects to defer alarplasty at the time of the rhinoplasty, but later decides that there may be some benefit to do alar base reduction.  I would wait 6-12 months from the original rhinoplasty. This allows for some skin and soft tissue healing to occurs first and facilitates the planning and execution of a secondary alarplasty. 

Alarplasty if needed, is done, most conveniently, in conjunction with rhinoplasty. If there is a doubt and not done at that time, it could be done, probably, within in a few months.