Placing the OS LFU near wall will act as additional horn path which can increase output some on the low end. Best to measure because in some set ups it will be better corner loaded and in some better as assofi1 described. Depends on room acoustics. In my room corner loaded was better.
as far as balancing the OS with other subs, I have had multiple configurations and when I had one OS LFU I was running it with an Epik empire and two velodyne SC-12's sealed subs. The main issue is different phase repsonses over the frequency range but it can be done with some systematic measuring and tweaking of independant time delay, XO type and slop and phasing.
When I went to all OS LFU the job became easier because the all have almost identical FR and phase alignment.
If the room needs different subs, then get a mini DSP balanced 2x4, measure with REW and you will be amazed what you can achieve.
Pre-Pro: Emotiva XMC-1, DSP: Mini DSP 2x4 balanced
Amplifier: Sunfire TGA7400, Lab Gruppen FP10000Q
Speakers: JTR Noesis 212HT (LCR), JTR Slant 8's (surrounds), 3 Orbit Shifter LFU's
Sources: PS3, HTPC, Dish Network
Projector: Epson 8350
Screen: Seymour XD AT 138" diagonal 16:9