You are going to get three 1099s, will drive them with 300 to 500 watts each from a stack of four PA amplifiers in an apartment.
....and you don't want anything "insane"
We like the cut of your jib--Welcome to AVS!
The traditional answer would be the Volt series surrounds be in 6.5"/8" or 10". They are coaxial drivers that are quite efficient at 93dB one watt/one meter for the 6.5" and 95dB 1w/1m for the 10" version. Running a pair of Volt 10's on the XLS1000 would get you rock concert levels---I mean "headroom" to spare. The kits run $125 to $200 each depending on if you get flat packs, type of options etc.
If you desire more power handling / efficiency, some people have used the Fusion 10 as a surroound. The F10 uses the same woofers and horn/compression drivers as your 1099s if that is something you care about. They are 98dB 1w/1m and a pair of them with flat packs would run around $500. Another option is the 88 Special, it is 98.5 dB 1w/1m, handles more power and is only around 8 or 9 inches deep although quite large in the other dimensions.
As far as what you "need", the Volt 6 would be great for surrounds in an apartment. The smaller drivers have better/wider dispersion which works well in smaller rooms. Volt 8's would give you a little more power handling and output while not getting too large for the wall--the Volt 10, F10 and 88 Special I would make a cardboard replica of the things to give you an idea of how large they are and what options you have to properly place them.
Good luck and welcome to AVS--you are too far gone with a rack of PA amps in an apartment so no use warning you about the rabbit hole.