Perhaps a better definition of "we" and the visons "we" have for the layout would help.
There have been mulitple scale layouts built - perhaps the best know one is/was a club in the East Bay of San Francisco whose O, S, and HO layout was featured in Model Railroader in the 1960s. Each scale was more than just a display run. The O was generally lowest and closest, with HO being the highest and furthest from on-lookers.
If we is really an I or just 2 people, time is often the resource with the most constraints. There often isn't enough time to do a single scale justice in the space available, much less three. In this situation, one scale tends to dominate and the other 2 portions either never get finished or end up as display layouts. This even happens in single scale, multiple gauge layouts (like my HO/HOn3). I ended up cutting my planned standard gauge track back considerably as the narrow gauge interest started to dominate.
In the East Bay club mentioned much of the S portion was the work of just one person. If that person were to find other interests, who would maintain and continue the S portion of the layout?
Obviously, the biggest problems in designing such a layout are going to be access issues and things looking "wrong" from a scale appearance perspective. If the latter is not an issue for the "we", then the design becomes much simpler.
Which is why I asked about the collective goals and visions for the layout. Putting several visions together club-style is difficult. Multiple scales will make it even more difficult to achieve a unified vision and purpose. Writing this down in advance for the group of builders to share is probably my top recommendation.
my thoughts, your choices