And would it have been uncommon for a group of about twenty-five Vikings to go off and raid villages on their own? Or did they always travel in the hundreds/thousands?
Did Vikings raid in Scandinavia? All the time, for sure. Keep in mind that it is only at the close of the "Viking Period" (say c. 1100) that we really begin to see full-fledged kingdoms or what we can really recognize as Scandinavian "states" (and I use the word very loosely). Throughout this period various groups of pirates and raiders would certainly prey on other Scandinavians. We see several instances where the losers in political struggles actively raid and attack the "kingdoms" of the winners. So, for instance, Harald Klak, an exiled Danish "king" and his descendants spend a lot of time raiding both in Francia and in "Denmark", where they have been excluded from the throne. The Vita Anskarii tells of similar activities in Sweden (see esp. chapter 19). So yes, there were Vikings active throughout Scandinavia as well as abroad.
Numbers: It was actually probably much more typical (especially during the first part of the Viking period, say c.900-c.960) for Viking groups to be relatively small, a few ships at most. The ability to strike quickly and to hit targets where little resistance could be offered was far more important than numbers. It is only later that we start to see what we might consider "armies", especially once groups of Vikings began to stay in Francia and England over the winter rather than returning home. Large armies were atypical until the second half of the 9th century. The so-called "Great Armies" that ravaged England and Francia in the 860s-80s were remarked upon for just this reason.