There is no point in changing all the strings, especially since the remaining ones are so new. When comes the time for total replacement in 6 months, a year, depending how much it gets played, then yes, change them all.
I'm pretty sure singles are available, but if not, a whole set isn't going to break the bank, and having spares is a sensible situation to be in. Bit like that insurance policy - useless until it's needed!
As a new violinist, be very careful you tune to the correct notes, and not an octave higher. Seen it happen, not a pretty sight.