Wouldn't it be true of any trade that both teams have to stay under the cap? I agree with the understanding that trades can only be executed after the season begins.
May I suggest a short period right after the draft is complete as a Trading period; that way owners could set up their rosters before finalizing and sending them in.
Per league rules, trades can continue after the season begins. Just for the record, I wouldn't have any problem with the Foxx/Fisk trade. Also, I don't know about vetoing trades. Other owners don't get to do that in real life. The commissioner could potentially step in if there was a hue and cry. As always, just mho.