I've never quite understood what qualified for SiC time... if I go up
with a friend, and both of us have the ratings and such applicable to
the aircraft, does one person go PIC and the other SIC? Or can I only
do that if the aircraft is required to have 2 pilots?


The plane, or the operation, needs to require two pilots. Just going up with a
friend does not. Going IFR with a friend does not. Going IFR in the soup with
a friend who is not IFR current does not. However, putting on the hood for IFR
practice in VFR conditions DOES, so the safety pilot can log SIC time. (this
is written up in one of the FAA FAQs, though those FAQs are not regulatory)


Also, I've been logging the time I fly with my dad, even though I'm
not technically able to be PIC (no tailwheel signoff). I made up a
category I called "second pilot", since I figured that I was gaining
experience and all but I couldn't put it down as SIC. Could I have?


No. You were a passenger, even if you were manipulating the controls. (That
is, unless your dad is a CFI with the proper endorsements AND he was giving you
flight instruction).

Jose

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