I use a 28" normal bike myself. But you can adjust the castor by quite a bit, so it should also work with a smaller bike. The rear wheel axle will probably also be a "normal" one, there is an axle included in the delivery, which you have to insert so that the hubs fit on the holder of the stand.

A Bluetooth heart rate monitor, I have the one from Wahoo: https://www.galaxus.ch/en/s3/product/wahoo-5032216-bike-computer-accessories-8859633, an iPad, iPhone or Android device, or computer and an app, i.e. Zwift or the one from Tacx. It worked well for me with Wahoo HRM, IPad and Zwidt.

To connect it to the PC it simply needs ANT+ or Bluetooth Smart, the Zwift app can be connected via Bluetooth

For the slope, yes with the zwift or tackx software (max. 7/8%). For disc brakes, I don't know.

The axle must be replaced. However, an axle is supplied by Tacx.

Mine, bought March 2016, came with the qr sqewer.

The scope of delivery includes roller, front wheel support and interchangeable tensioner

I wondered why (liability reasons?) but I always change them and install the original axle for the road. Works fine.