>>292628585
The unity collab thing is actually free for 3 person for up to 3GB project size.
[spoiler]I have a 25GB account if you are ok with us using that.[/spoiler]
Another advantage of the unity collab is that it's built-in compatible with unity without any glaring problems and just works without any problem. Only problem is version control. A problem that also occurs if we use git.

1) Make sure to save scene often and version v0.10, v0.15, v0.24 etc...
2) All files if possible is not deleted unless very sure is not used in the latest few scenes.
3) If we're going to change or implement something modular creation is best, so for example your physics system is on a separate prefab scene from the final version.
This will ensure that when I update the scene I don't overwrite your physics update. Using proper use of modular prefabs circumvents this problem. 
4) All prefabs are shared across scenes. So if one prefab is updated it updates everything. Change prefabs very surely and carefully. If we're to change something big duplicating the prefab and renaming the NEW duplicated one as a back up. Continue working on the original file

OR

We can just also coordinate that only one of us works on the whole project at any given time. That is the simplest way to ensure nothing wrong goes wrong. Been doing collabs for quite a while now and all the heart aches along with using collabs.