yes, of course you can play back samples at arbitrary rates using BufRd (direct phasor control, like poke~) or PlayBuf (trigger-based, like groove~.)
for streaming arbitrary length files from disk rather than RAM, you need DiskIn (VDiskIn also performs resampling / rate changes.) for this you provide a streaming buffer and you can keep timing tight by explicitly cuing files to the streaming buffer as appropriate.
the Timber engine demonstrates both from-disk and in-RAM methods and may in fact do everything you want to do already.
i continue to encourage people to use supercollider’s facilities for sample playback instead of softcut, if you do not need varispeed writes and crossfades. crossfades are possible to do in supercollider but it can be a headache. varispeed writes with crossfades are basically impossible to pull off in vanilla SC. (believe me, i’ve tried. hence, softcut’s existence.)