That is incorrect.
If you acquire a player that was on a roster last season, they can only be listed as a Keeper. So if you acquire a player via a trade then that player [b:536c9a6365]must[/b:536c9a6365] be listed as a Keeper, and you can only have 8 Keepers. So if you already have 8 keepers, then you would need to cut one.
Also, please note that once the FA Draft has started, [b:536c9a6365]any & all[/b:536c9a6365] players that were on a end of season roster are now considered Free Agents, and can no longer be acquired via trade.
Furthermore, once the FA Draft has started, to avoid confusion, and unnecessary complication, you can no longer cut players from your keeper roster. However you can still trade a keeper(s) for another owners keeper(s), so long as neither team goes over 8 keepers.
Lastly, once the FA Draft has begun, while trading will still be allowed, it will need to be properly balanced. As I said before; as long there is no major objection from the league, all trades should go through.
So something like trading a 6th round pick for Jake Peavy would likely be disallowed, as would drafting a player with your first round FA Pick and then trading that player for a 5th round pick.