Hey there!

I am the proud owner of 3 Southern Leopard frogs. I'll be happy to answer your questions.
1.) Leopard frogs aren't poisonous per se...but like all frogs, it's recommended to keep leopard frogs with leopard frogs, because even though their skin isn't toxic to us, it can be to other frogs.
2.) They're relatively easy to keep. You want to make sure your cage is about 50% land, and 50% filtered spring water (chlorine is a silent killer for these guys! make sure your water is filtered spring water). Some people set up semi-aquatic tanks with a filter, but I personally use two large dog food dishes. It's easier for me to keep clean for them.
3.) I don't use heating on my babies. They like it room temperature.
4.) Along with the 50/50 set up, I just use crushed coconut substrate along with some fake plastic plants. They will burrow (I didn't believe it until I saw it, but they do!) so you want to make sure their substrate is fairly deep. You can also add a log for a hiding place for them. I guess it's more of a forest set up?
5.) The size of the tank depends on the frog. I believe Petco sells Southern Leopard Frogs, which is good because they only get to be a few inches. A 10 gallon cage would be good for one, and then add 5 gallons for each additional frog. I'm not certain how much the cages sell for at Petco. I tend to get mine used from a local pet store out here for around $20 for a 20 gallon.
6.) Answered above.
7.) They can be quite active! Sometimes when I am trying to sleep I hear them splashing around. Sometimes my male also calls a bit, but he's not terribly loud. They're most active around night time, but I've seen them hop during the day.
8.) Southern Leopard frogs are around 3-3.5 inches at adulthood. Northern Leopard Frogs get bigger...closer to 5 inches I think.

I hope this helps with your decision!