I thought XPs only come from Player Soldiers...am I not remembering that right?
Since player soldiers are the only ones that can earn XPs then is stands to reason that they are the only ones the XPs can come from.
However, any player soldier can cough up the XPs to pay for any card, at any time, even during squad assembly.
oopsActually from the manual
Non-player soldiers gain and spend Experience Points in the same way as Player Soldiers
And the same text is in the Squad Soldier Cards section