I haven’t personally played a skald before (either PNP or in WOTR Alpha), but morale bonuses to strength and constitution (mainly strength) will work reasonably well for that party. The conventional wisdom is that skalds are for melee parties that like strength, but both archers like strength (for composite bows) and Sosiel will like use a 2H glaive (which also likes strength). I suspect, however, that you’ll get slightly more from a bard overall because the competence bonus applies both to-hit and to-damage, which will help Ember for ray spells (a skald will not), be better for Lann/Aru because of the to-hit bonus, be equal for Seelah, and slightly worse than the Skald for a 2H Sosiel because of the bonus % scaling on strength for power attack and a 2H weapon.
You’ll want a trickery character, if you don’t include Camelia or Woljiff, but that won’t be difficult w/ backgrounds. Or you could just skip every locked chest and door and feel bad for the entire game.
I personally prefer a 2nd melee character with high AC, if my party lacks an animal companion or a reliable summoner. Sometimes you’ll have combat where it’s front & back; sometimes your first tank will die or be feared or miss a save-or-suck (although your chance of failed saves is low with a paladin); and sometimes you want team work feats for your melee (outflank, shield wall, etc.). Lann might be able to Legolas-tank with a dexterity after point blank master–although I haven’t tested it myself–and a Skald might be able to have a high enough AC with mithril medium armor and enough mirror image/blur/DR from rage. But there’s some anti-synergy with a skald tank because he’ll want to stack strength to capitalize on his buff, but he’ll need dexterity to boost his AC. Again, a bard is probably more flexible because she could stack dexterity, grab fencing/slashing grace, and still benefit from the to-hit and damage buffs of her song. Alternatively, you could have Ember or Sosiel function primarily as a summoner and just roll with Seelah.
I’d encourage you to consider Nenio over Ember. Without rehashing the “are witches useless” debate from another thread, witches miss some useful arcane spells. The most important of these is haste–which a bard or skald can provide. I’d suggest you look at the 2-3 or so arcane spells that you really want to have from lvl 1-9, and then see whether bard + witch leaves gaps.