Tried to install Sparky Xfce Stable from a live usb to an exisiting ext4 partition, but got extra imaginary free space partitions showing in the 'After' section (like in the link below) - so cancelled.
I believe this may be a calamares bug as explained here: https://github.com/calamares/calamares/issues/1194
I don't know if anyone's interested, but the calamares installer problem described above applies to the Mate rolling version too.
However, I have found that if you delete the partition you want to install on, to make it 'unallocated' the 'Replace a partition' function seems to work properly (no imaginary partitions created).
Presumably the install will then go ahead without problems (I haven't actually fully installed).