It is true. Also when the stem of a feminine name ends with the letter "L" we use "-u" ending in the vocative:
Ela - Elu, Ola - Olu, Mariola - Mariolu.
With other feminine nouns ending with "L" it seems it's always "-o" like in "kula - kulo" (correct me if I'm wrong please.)
I think that these are in the imperative mode, correct me if I am wrong, explain to me why they are vocative please.
The vocative case is used whenever when you talk directly to some person. The imperative mood is just one of the examples of such situation. Others could be:
greetings - like "Dzień dobry ciociu"
yelling insults - "Ty debilu!"
questions - "Mamo o której wraca tata?"
"Szeregowcze, sprzątaj po sobie!"" is correct but you may see "Szeregowcu..."
Both forms sound correct to me when you talk about the soldier. "Szeregowiec" however has two meanings - "private" (in the army), and (I don't know how to name it) "a house which is standing in a row of houses which share their side walls". (you can google an image to see what I mean)
In the second case you should only use "-wcu" ending as the "-wcze" ending is used only with masculine people.