We have three turtles (RES, Florida Redbelly, Peninsula Cooter) in two largeish tanks (125 and a 150), here's my experience over the years with your questions:
1) I always wash my hands and usually use hand sanitizer after doing work on the tank and/or handling them. I also avoid touching my eyes, nose, mouth, et cetera before washing. If you are reasonably careful, I suspect the chance of salmonella is very low, as I haven't even heard of a case among other turtle folk (I'm sure many doctors have though). I don't use gloves while I work with any of the freshwater tanks, but it obviously wouldn't hurt.
2) Ours were from breeders, which may or may not help with #1, but I'd still strongly suggest getting a captive born one rather than wild caught for the usual reasons.
3) I had an XP3 on the 125, and the physical media (sponges that come with it) couldn't keep up once the turtle in there got to 5" or so, and would clog rapidly. I'd suspect two would be able to do it if you don't mind rinsing the sponges, but I just moved on to sumps with filter socks, which I would strongly suggest if you can do it. I use matrix for the biomedia, and never had a problem with the biological filtration.
4) I use the same aqueon changer for the turtle and non-turtle tanks; haven't had a problem, but if you are worried, it's not like a second one would break the bank.
5) Never used reptoguard, and the turtles have been healthy for years.
6) At the current house, I use the outside spigot and drain into the yard, but previously I used the kitchen sink, and would wash it mildly afterwords, again without issue.
I'm assuming you already know, but UVB and a varied diet have been the easiest parts of keeping ours happy & healthy. Also, I strongly suggest pool filter sand or something similar if you are using a substrate. Gravel has a reasonable risk of non-resolving impaction, especially with younger/smaller ones.