@sicelo:
No, it doesn't oblige you to overwrite anything.
You can use Safestrap - modified TWRP recovery - in order to manage so called slots where Android OS ROMs and Sailfish OS can be installed.
You install the Safestrap from stock ROM but after installing it you can get rid of stock ROM.

There are several types of ROM slots available in Safestrap recovery.
Firstly, there is the stock ROM slot which uses partitions used by stock ROM by default.
Secondly, there are 4 slots which use loopback devices located on internal memory (in /sdcard/safestrap directory on any Android ROM).
Thirdly (or rather - sixtly), in modified by @Internauta37 Safestrap recovery available here there is the 6-th slot available. It uses "abandoned" webtop related partitions. Webtop was a Linux desktop environment available when using stock ROM in version 2.3. Later, at Android 4.0.4 upgrade, this feature has been removed by Motorola but the mentioned partitions are still available and it would be kind of waste of to not be able to use them. But be aware this slot is not supported by CyanogenMod/LineageOS by default and you have to modify ramdisk.img file in order to install ROM here.
In recovery modified by @timangus the 6-th slot is called altpart. This slot uses /webtop partition as /system and rest of partitions are the same as in stock ROM slot (it means it conflicts with stock ROM slot so you cannot install ROMs on both of them at the same time). It was created as /webtop partition is bigger than /system and installing for example LineageOS 14.1 leaves few space for installing gapps on stock /system partition. But be aware this slot is not supported by CyanogenMod/LineageOS by default and you have to modify ramdisk.img file or install mentioned on the website zip file in order to install ROM here.

So, in total there are 6 ROM slots available. You can have 6 ROMs installed at once (including SailfishOS) and use Safestrap recovery to switch between them.

Although, I am not sure how installing Maemo Leste affects Safestrap etc.