Middle name for Penelope and family politics

[Bloom]

So we have pretty much settled on Penelope as a first name. Nicknames may include Penny, Pen, Nel, Nellie or Poppy. But we are still undecided on a middle name.

DH's mum's middle name is May. I like this name. No one else in the family has used one of MILs names.

DH's childless aunt's name is Diana (whom we really admire and she is lovely and very involved as ours are her surrogate grandchildren).

I wonder if MILs nose will get out of joint if we use Diana and not her name. Can we use both or it is too long? I am normally opposed to 2 middle names (for us). This may well be our last child.

Our last name has 3 sylables (sp?) and ends in y.