First, thank you!!! JD!! Your tip about the "pusher" was essential for correctly inserting the tampon properly. My fingers would not have worked. I used the tip of a fleet enema, which was just the right size to push the tampon in properly. I was initially surprised how big the tampon was, but again, JDs advice about plenty of lube saved the day.
I am trying the small, and once it was inserted, could not feel it at all. However, after about 15 minutes, I began to get strong urges to defecate. These continued for several hours, although I could only feel the "string" itself. Looks like it was just the gas inside trying to get out (it did leak out and that reduced the pressure). I think I will purchase a box of the large and alternate to see what works best. As of now, I don't have the confidence to wear them in public, but my experiment at home is so far successful.
I will post more as I continue to experiment. So far though, they have eliminated the mad rush to the toilet I was getting after eating. I have a Lower Anterior Resection, so my descending colon is directly connected to the backdoor (meaning it is much smaller than the original equipment, with less room for storage). The Dr. says that it will eventually get larger and reduce the pressure, but first I need to tighten up the sphincter (if possible). I will say that I am really able to target my abdominal floor exercises really well with this in place, so far I have had to tighten up on my sphincters multiple times each hour just to combat the urgency to push.

Well, this is only the first day, so I will wear them for a while and report back about how they are working. If I can gain confidence to go into public after eating, it will mean the world. I really want to go to a restaurant again, it has been so long.
Mike