Risk of evisceration does go up in foals under 3 months old (there was a study, but I wasn't able to find it quickly). The inguinal ring is not 'closed' (the ring never really closes) at birth and becomes tighter at about 4 months after birth.
I have had 3 eviscerations in my career. 1 was a crypt-orchid and the other 2 were in foals under 4 months. Certainly not enough to consider a study, but enough that I now suture the ring as well as the cord (I have always sutured the cord). Unfortunately, although the sutures never fail, the tissue they go through is not strong enough to hold back the abdominal contents and does fail at times.
The good news is that minis are less likely to have 'open' inguinal rings than are draft horses and standardbreds.
My $0.02 worth on the subject is I am less concerned about the risk of being under 4 months done properly and more concerned with the idea of the 'Sunday Gelding Clinic'. I assume this Clinic is being conducted to encourage geldings by offering a reduced rate. As a consumer I would have to wonder what corners are being cut to achieve this reduced price? Is it the anesthesia? the pain management? the environment? the skilled labor? the suture? are instruments going to sterilized between surgeries or are they going to be quickly washed in a detergent and used over and over? will a new scalpel be used? are syringes reused? is the Vet prepared to handle a problem with additional general anesthesia and instruments?
If this colt isn't of much value to you monetarily or emotionally to me, then I get seeking out a cheap price. But, if I liked this colt then I would want the geld to be performed in as risk and pain free way as possible.
Dr. Taylor