From scalability perspective, it's best to jump directly to commercial MFH (5 units plus).

I look at investments that are scalable and not speculative 

The challenge I think he will face going with several 6/3's over a MF is that renting by the room is management-intensive, hard to scale, some areas have occupancy limits, and the biggest challenge which contributes to the scalability is probably that there seems to be extremely limited inventory of 6+/3+'s or even 5/2's with basement apartments for under $500k anywhere in Denver metro (correct me if I'm wrong Craig!). 

I'd like to do MF, just because of scalability; but for a first purchase, I'd be okay with a SF and renting out by the room.

Thus the business model is not scalable. 

One of the benefits of large multifamily is the ability to find professional management  it’s because of this that it is more scalable 

I am a Podio developer and have helped tons of people on BP to get their system up and running and scalable.

It’s affordable(not free version) and scalable to grow with your enterprise.

I'm not at the point of apartment investing yet, but I have a bunch of 80-100k properties and though the returns are really good, this just isn't scalable. 

Finally, it's just not scalable.