Thanks! HDMI cable length is pretty limited, and unfortunately there's no hard and fast rule. It depends on the cable itself as well as the strength of the signal put out by the camera. Generally you won't get more than 20 feet in the best case scenario though.
There are some active HDMI cables that can amplify the signal, or also HDMI repeaters, but it'll probably take some trial and error to find ones that work with your cameras. The "pro" version is to convert to SDI and back, which can go up to 300 feet.
Yep! The Decimator both converts from HDMI to SDI and also scales it, which is handy on the larger ATEMs that don't handle scaling themselves. On the other end, you'd need SDI->HDMI back in to the ATEM Mini if you're doing that.
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Yeah exactly. You might shop around a bit too cause sometimes the one-way converters are cheaper. Both those devices linked will convert both ways, but you could use a separate HDMI->SDI and SDI->HDMI device if that is cheaper.