In the in the History of Savo (Kauko Pirinen: Savon historia II:1,
Kustannuskiila Oy, Pieksamäki 1982, p.214)I found something very
interesting for you. Pirinen mentions that a cavalryman Sigfrid
Pentinpoika was in 17th century a leaseholder of the farm Kasanen in
Kerisalo, and used name Ruth. Pirinen also says that the Kerisalo-farm
remained thereafter in the hands of the descendants of Sigfrid. Why did
Sigfrid use the name Ruth (Ruuth) I do not know. There are several
Ruuth-families in Finland. One of them lived in 16th century in Viipuri
and was very rich. The Ruuths in eastern Finland might be their
descendants, but as you said Ruth might be a soldier name too.

the above material from:Jouko Salminen

Sigfried Bengtsson should be the first then...
but there are no Bengt Ruuts, Sigfried Ruuts.
there are no Sipiläs before about 1750