Unsure if you are referring to the source (SMG) or the product as "trusted source;" however I have orderd from SMG plenty of times - just recieved a package yesterday
As for the holster; SOB holsters and larger-framed handguns are not a good match. Sure your Glock would fit, but you'd have to worry about stress on your spine/lumber-region with the weight and hang of the unit (holster and Glock). If your intent is to have a multi-use holster; I'd also be wary of that transitioning as well. Mostly, in a tactical use situation, you want your hand and reflexes to run "automatic" to your weapon, draw, aim and if necessary fire - as smoothly and fast as possible. If you have your holster in multiple positions, it may not work as smoothly and fast as you may need for it to be.
SMG does have a very good return policy, and your "investment" here is about +/- $40 bucks; I have yet to have or need to, return anything to SMG, the products are of reasonably quality; not high-end, but functional.