So I just re-read the title of this thread. You want to use this as a combo plotter/FF? That I wouldn't recommend if you plan to split-screen it. Have two tiny 3.5" images next to each other would be kinda tough to use. Running with one or the other full-screened would be O.K., but as a split-screen instrument, not so great.
The way I fluke fish I have the Garmin 7" plotter tracking my position in real time and the 8" Lowrance FF watching the bottom. I can see both from where I fish on my rear deck - but, having a 9" plotter and a 10" FF would be infinitely better - I do have the room, but I'm not replacing perfectly fine instruments while they are problem-free. I'm cheap that way.
I know you say you are "room-challenged" at your helm, but if you do go with a 7" screen, I would plan on using one function or the other at a time, if you do want to see what's going on around or under your boat from any kind of distance at all.
My current unit is a 20 year old Lowrance LCX 18c combo unit
In a perfect world, I am trying to keep that in place and add the Garmin unit so I can have 2 machines working at the same time