Jobspopo, yes puffers I find are quite indifferent to most fish regardless of size, they seem to just bumble around in their own little world quite contentedly. That's part of the charm of them I guess.. My fahaka does the same (most of the time at least).
Thanks for the Intel, I will have a look for the intermedia as my lfs is pretty good.. But it might be a stretch lol
Further more, do you have the hydra system attached? I have one on my tank and it really takes the edge off the nitrate build up. The moe large fish you have the more effective it is.