If ACE is allowed for SMB3 and SMW, and All-Stars is allowed for the first 5 2D games, then would the All-Stars category "Any% ACE - All Five" (5 games in less than 10 minutes) be allowed? Or would it be counted as "doing multiple games at once"?

"Multiple games at once" is more referring to the concept of viewing all games in a given Multi Mario run as individual runs done back-to-back in a marathon setting. We haven't had a proper discussion on this yet but I'd argue that you shouldn't play a different game to achieve completion in the game you want to complete. For example, using SMW to achieve completion in SMB2 as shown here by SethBling: https://www.speedrun.com/smb2/run/z5vgd1nm .

ACE in SMW and SMB3 only require these games themselves (and in the case of SMW maybe more hardware like a second controller, this is the setup I use for 0 Exit for example: https://docs.google.com/document/d/10q--6Cmv_j4ZQTkdzm1qELzHNxGgG8wSqZ074nk19Rc/edit ) so they don't come into any conflict with this specific rule.

So in short: you would need to set up ACE for every single game if it is ok to do so. Otherwise this is not fulfilling the criteria of not playing more than 1 run at the same time, as you use the same ACE setup for all the different games, meaning you are doing multiple games at once.