Here's what I'd do:
Since you've got roms for .127, give the oldskool games a shot on MAME 127-you just might find that the newer builds work just as fine as the old ones. most of it depends on your CPU power. You might never look back from here onward.
Make your game list with either malaGameList or ROMLister - Put all the games you want into that list, although ideally, you'd only have a list of games that are actually playable on your machine (ie, no trackball? don't have centipede in your list) Romlister will help with that.
Then, playtest your .127 MAME+romset. If you find that mame 127 doesn't play a game as well as your old .79 did, then you have to intercept the launch section of when MALA tries to kick off MAME. That's the stuff that is described in that link. Essentially you tell mala to launch mame.bat not mame.exe. Then in mame.bat, you examine the name of the game that is about to be launched. At that point you can do anything you want, including switching to a different folder to launch a different version of MAME.
So you would have your folders set up like this:
c:\mame <- your .127 mame goes here
c:\mame\roms <- your .127 roms go here
c:\mame\mala <- mala itself goes here
c:\mame\79 <- your old copy of mame .79 goes here
c:\mame\79\roms <- your .79 roms go here
In your c:\mame\79\roms, copy only individual games that you want to "override" from being played on your .127 build.
The batch file then examines the game that is about to be played, sees if it's one you want to override. If so, switches folders to where your old mame.exe is and launches mame.exe from there.