Hi!
Some info on the "new" frames.
The first new frames from Mellotron Archives were made in 1993 and Dave was selling them in late 1993. The MkVI was introduced in early 1999.
I do not know what exactly JK means by "Older tapes (8 sec.) will do fine in new frames, but not conversely", but the new tapes from mellotron.com have been working fine in the old frames since 1990, and continue to do so. Of course old tapes can be fitted in the new frames.
Concerning the original question: Since the tapes from Mellotron Archives are delivered with extra length it's very easy to adapt them to the needed playing time, up to 10 secs in a MkVI frame. Of course that only makes sense with the sounds that are that long, like the Oboe for example.
Longer tapes make both old and new frames a bit less convenient to handle, so it's basically up to the user to decide.
Best
Markus Resch
Mellotron Archives