Hi, I just ordered a three piece Be23 set. I apologize for being long winded with this. 

I have a few questions. I would like to add this a preexisting network. I've got network experience but have not messed with a MESH setup before. My understanding is that this MESH replaces existing networks instead of adding onto them. I really don't want to redo my entire network.

Almost everything is currently hardwired. 2 windows PC, a network printer, a network scanner, a game console and NAS. Attached via wifi is a laptop, android tablet and my phone. All cables are cat6.

My current set up is in a 2 story house. I have the first floor and my relives have the 2nd. The stairs going up is near the center of the house so any wiring has to fork, daisy chaining is not really an option. I work from home and my work pc has to be hardwired I can not use wifi as per my employment contract. 

I've got a fois gigabyte plan. I called and asked and they don't do modems anymore. The line comes in to their router. from the router to the living room to an 8 port switch (tl-sg108e). I also have raspberry pie running pie hole for my DNS attached to the fois router. Almost everything on the 1st floor is attached to this switch. I would really really prefer not to change anything on the 1st floor. 

From the switch it goes to 1- mini pc attached to my tv as my main entertainment system, 2- game console attached to same tv 3 - my home office containing my desktop and a raspberry pie working as a print server for my printer and flatbed scanner. 4- the NAS, 5- a line going up the stairs to a 2nd switch (tl-sg108e), I know it is a bad Idea to stack to many switches. It's only 2 and give them a few ports to plug things into. 

the 2nd floor is where I'm having problems. The house basically 2 apartments and we share utilities, I own the place and my brother and wife live upstairs. I want to use this entirely upstairs on the 2nd floor, they prefer wifi and I agree as I don't want to pull wire up there. I currently have an extender up there but it doesn't work all that well and am hoping this MESH will work better

Can I use this mesh without redoing my entire network? Can I segment my network via the switch and use this mesh on one of the segments and not the other? Should I use router or access point modes and are there pluses and minuses to each?

Any advice on configuration is greatly appreciated.