I don't own an AR myself, but as I understand it, yes, you need to make sure that 120/sample count is a whole number. Hence the "Add silent slice" button to add extras.
In Grid mode the program automatically adds silence to short samples so that all are the same length.
The program does not do any sample rate conversion. So if you, say, select 22kHz output from 44kHz source files it will play back slower, not get reduced in size. I did consider adding this, but I didn't trust I'd be able to do it as well as professional software, so the sound quality would probably suffer. Instead the output data remains unchanged except for mono/stereo conversion. From what I understand, the AR automatically does sample rate conversion, so probably best to just keep sample rate the same as your source files and let the AR handle it.
The OT attributes does nothing for the AR. They are only used by the "Normal" and "Steps" modes, which does not make sense for AR owners anyway. These modes create a small .OT file that contains these settings and "slices" for the Octatrack, meaning all samples in a chain doesn't need to be the same length, as they need to be on the AR. In these modes the "Add silent slice" button adds 1 second of silence. Perhaps to be used as place holders or something. I was thinking cases like a chain with kicks starting at slice 1 and snares starting at Slice 32. Then silent place holders could be used so that new kicks can be added later on without affecting positions of existing samples.
Thanks for the XP info!