As far as ecological concerns that has a lot of variables that goes with it. You have to think about where you are in the country, what the population of Garters is like in your area, State laws about collecting wild animals for private use, and also if you are worried about an ecological impact what gender you are going to collect. For example, taking a female out of the population will have a greater impact than removing a male.
When it comes to parasites there is a relatively good chance of there being some form of parasite. The best thing you could do is keep it separate from any other snake you have and monitor its health. Typically it isn't a very big deal for the wild Garter as they have lived with it, more than likely, for a long time in the wild.
Just providing a comfortable (i.e. hides, water, etc.) enclosure should help reduce stress in the animal.
The eating issue is on an individual basis as they all have their own personalities. Some will take to it faster than others.