I was able to access the following online article about the effect of spaying and neutering on the incidence of ACL injury:
Canine Ovariohysterectomy and Orchiectomy Increases the Prev... : Clinical Orthopaedics and Related Research
Here's a table from the article (sorry for the formatting):
TABLE 1. Prevalence of ACL Injuries
Gender, Number in Sample, Number of ACL Injuries, Prevalence (%)
Female
Sexually intact, 742, 18, 2.43
Spayed, 1067, 55, 5.15
Total, 1809, 73, 4.04
Male
Sexually intact, 861, 18, 2.09
Neutered, 548, 21, 3.83
Total, 1409, 39, 2.77
Male and female
Sexually intact, 1603, 36, 2.25
Spayed or neutered, 1615, 76, 4.71
Total, 3218, 112, 3.48
It looks like the rate of ACL injury is approximately doubled in neutered males and females. Note, however, that the rate of injury is still low, and that one cannot prove that neutering was the cause of ACL injury in any particular animal. Also, these are the rates in dogs that were seen by an orthopedic specialist, so they are prescreened for having some sort of orthopedic issue.
I will look further into this article and post any additional findings about the rates in small dogs, if I can find any.