May I offer you up some of your own advice here?
"Go at the speed of the slowest person"
Yes - you and Mono have been dating for over a year, but this particular situation must be incredibly new to Nerdist.
Obviously I cannot possibly understand all of the dynamics which come into play here, but 20 months is arguably not very long for someone to become a part of your life in a permanent way, let alone move in with you and an already established family dynamic.
I'd give Nerdist some time - not backing away from Mono, but simply letting the chips settle post Rolypoly, with your new relationship with Derby, and the idea of Mono moving in. You don't need to set a date on which things should be settled. Setting a time-line creates pressure, something that can be inhibiting for trust building. So long as things are discussed and not dropped while also giving time for each other to breath and process, I think it is entirely reasonable.
There is a lot going on for you, don't you think?
Maybe sit with it for a while?