Both those panels don’t have complete specs, and the pictures aren’t very high resolution, so it’s hard for me to tell without having access to more details or the panels themselves to know if they will be supported. On one of them I can see at least the shape of the silkscreen letters around the input connector, and it looks like “HUB08” not “HUB75”. HUB08 has only half of the signals of a HUB75 panel, and I believe a different pinout of HUB75. You’ll probably need an adapter to take the HUB75 output from the SmartLED Shield and split it into two HUB08 signals to go to two panels. Something like this. I have a HUB08 panel, something like this one, but I’ve never used a HUB75-HUB08 adapter and don’t have one of those.
The 32x32 panel has two sets of connectors on it. I’m not sure if there are two separate 16x32 HUB08 panels combined into a single 32x32 panel, or if there is something else going on.
The panels I have are similar to this one, which uses thinner SMT LEDs instead of the individual THT LEDs on the panels you linked to. They are spec’d to the same brightness as at least one of your panels (7500cd/m2), but who knows if those specs are accurate. In addition to the LEDs being thinner, the panel frames are likely thinner. The 32x32 panel you linked to has two boards stacked on top of each other, one for the THT LEDs, the other for the SMT driver parts, and it’s likely pretty thick. Can you use panels similar to the 16x32 panel I linked to?
96x64 should be fine with the Teensy 3.6, but there are tradeoffs in color depth, refresh rate, and overall brightness as the panel size grows. If you already have a Teensy 3.6 and SmartLED Shield and a supported panel, you can try to get a sense for how it will behave by increasing the size of matrixWidth/Height until it matches your intended panel size, and just look at what you see on the one panel you have hooked up.