The sleeper heights are slightly different between the SL-500 O-16.5 track and the SL-100 Standard Gauge track, the SL-500 being slightly thicker, so you'll have to pack up the SL-100 at the joint. I've mixed the different Peco code 100 tracks with no problems.
'Code 100' means the height of the rail is 100 thousandths of an inch (so 0.10"). The only issues with different makes of code 100 track might be in the size of the 'foot' at the base of the rail, but I've found you can usually tweak Peco rail joiners to fit.
If at first you don't succeed, use a bigger hammer.
The only thing that keeps me sane, is the friendship I share with my collection of singing potatoes....
Never knowingly sensible!