I actually don't mind that trade though would like at least one more piece. Millsap is just 27 with a very reasonable contract and while he is a bit undersized, would greatly help with the teams lack of interior scoring, rebounding, and toughness. Harris has obviously fallen off a bit, but he did miss almost all of last year with an injury so it may be related to that (of course he isn't exactly durable which is a problem).
I'd probably do this
Rondo, Allen
for
Millsap, Harris, Bell (depends on if C's feel they need a SG to replace Allen), 1st (top ten protected and the better of its two unless 1 in top ten 1 is not OR if both in top ten then the worse of the two)
It does affect cap space next summer, but Millsap seems to me to be an effective use of the cap space and I think the C's would still probably have enough for a max or near max contract.