I guess I like the spelling
Wallace more.
Wallis is I think now generally identified with
Wallis, Duchess of
Windsor (her first name was actually
Bessie), the divorcée for whom
Edward VIII abdicated his throne in 1936. But of itself, there is nothing wrong with the spelling
Wallis.
You probably already know this, but just in case:
Wally can also be short for
Walter.
I could see
Wallace with those names. It's already an interesting mix of names, and
Wally /
Wallace seems to fit pretty well.
Wallace MatthewWallace MichaelWallace AndrewWallace JamesWallace RobertWallace PeterWallace JosephWallace Alexander Wallace Daniel Wallace David
Since
Wallace is somewhat unusual and has something of a 'full' sound when said, I would keep the middle name on the simple, but still strong, and traditional side, to balance it out. :)