Is it possible to use the RTC1 and app_timer simultaneously at nrf51822 with softdevice?
app_timer uses RTC1, installs an IRQ handler for it, so you can't use RTC1 for a different purpose at the same time as app_timer, unless you modify the app_timer source.
RTC1 is not a restricted peripheral so it can be used with the softdevice and indeed many of the examples use app_timer.