I think the legals work the same way as the regular flat-rate.
And yes, there was a LOT of confusion earlier in the year about how much you could put in a flat-rate envelope. It is NOT true that they can only be 3/4" thick. But it might be true that some Post Office workers did not read the memos explaining that yes, if it fits, it ships.
You can put tape on the envelope too, including over the closing flap, and around all the seams. Even over the pull tabs. But I think you should limit it to only a single strip over the closing flap.