Question about Praying Mantis behavior

I work at an auto parts store in town. We recently had the exterior painted, and with the exceptional weather this week, we have had tons of bugs (mostly beetles) hanging all over it. So, 2 days ago I see a small browm Praying Mantis on the east side of the building. She was there for most of the afternoon. Finally come out to see that she is laying an egg sack, attached to the building. When she had finished, I caught her trying to walk into the parking area. I captured her and brought her home to my garden, where I thought she would be safe. Yesterday I found a MUCH larger Praying Mantis at the EXACT same place where the smaller one was the day before. What are the odds? So, in my ignorance of PM's, I am thinking, 'OH! This must be the daddy!'. Well, this one was not friendly AT ALL, and when I approached it, it reared up like a professional prize fighter! You could almost hear it saying, "Bring it on, humanoid!". I stood there, a bit insulted, but more concerned for my safety. Oh, I meant to say IT'S safety. Well, it starts walking into the parking lot, so I am thinking, "hey! Family reunion!'. I captured it in a foam coffee cup and took it home to same garden. Let it go in same spot, but little one was no where to be found. Went back 2 hours later, see big one (further into the garden) and just a little bit away is the little one! YIPPEE! Family reunited! UNTIL I went back about an hour later, and there is the big one snacking on the little one! Once I realized they weren't breeding, I got them apart. Little one was running across the dirt trying to get away. Big one ate some of her front leg off. I caught the little one and moved her to the other side of the house. Saw her this morning and she seems to be okay. Looked for the big one, but couldn't find it. If I do, I am going to relocate her about a mile down the road! My question is, is this common for PM's? I mean, what were the odds that I would find 2 different species (I have since learned they are NOT mommy and daddy, but 2 different species), in the exact same spot at the store, with nothing but concrete around? We are in the country, but still seems highly unlikely to find 2 of them in the exact same spot. Do females normally feast on each other? I think the big one was a female as well. Any info would be greatly appreciated. I can't seem to find any that addresses this particular situation. Thanks!!