Your proposed 42/38 chainrings are reminiscent of the old "half-step" gearing that was useful in the days of 5 and 6-speed freewheels and their wide gear spacing. It's a lot less useful with 8+ speed cassettes.

As bradtx noted, modern front derailleurs usually have a rather deep inner cage plate and wont clear chainrings that close together in diameter so that's not going to work.

Finally, what is the bolt circle diameter (BCD) that your 28T granny ring is mounted on? If it's the common 74 mm, a 24T chainring is the smallest that will fit.