Yes, if the aliens don't have enough "standard" spawn nodes (node rank > 0), then they will fallback to all other spawn nodes (node rank = 0).
PS: and if there aren't any other nodes available either (e.g. Xcom has already taken them all, because you have 500 soldiers in your base) and thus no aliens can spawn at all... the game will just crash.
/* following data is the order in which certain alien ranks spawn on certain node ranks */
/* note that they all can fall back to rank 0 nodes - which is scout (outside ufo) */
const int Node::nodeRank[8][7] = {
{ 4, 3, 5, 8, 7, 2, 0 }, //commander
{ 4, 3, 5, 8, 7, 2, 0 }, //leader
{ 5, 4, 3, 2, 7, 8, 0 }, //engineer
{ 7, 6, 2, 8, 3, 4, 0 }, //medic
{ 3, 4, 5, 2, 7, 8, 0 }, //navigator
{ 2, 5, 3, 4, 6, 8, 0 }, //soldier
{ 2, 5, 3, 4, 6, 8, 0 }, //terrorist
{ 2, 5, 3, 4, 6, 8, 0 } }; //also terrorist