Is сукенка unacceptable as a translation for dress?
Yes. It sounds slightly more colloquial.
So kinda like in Polish. In Polish generally dress is "sukienka", but a wedding dress or something like that is called "suknia". Generally Ukrainian sounds like older version of Polish with some Russian for me.
Nah, Suknya is definitelly 100% Serbian, though it means "a skirt" in ours