I think
Sidney is perfectly fine and even preferrable on a boy.
Sidney James is nice.
While I like
Morgan,
Sidney,
Mackenzie and
Madison on boys, I think it would be a little too much unisex for one family.
Morgan and
Sidney would be fine,
Sidney and
Mackenzie would be fine, but anything more than that and I'd probably wonder if the parents were trying too hard to fight the good fight against boys names on girls ;)
I don't like
Eartha, sorry.
A bad wound may heal, but a bad name will kill.
Scottish Proverb
"'Scuse me, do you mind not farting while I'm saving the world?"