Army has 4 — 14 slots depending on your rank. Only creatures in your army slots take part in battle. Warlord is the only unit you can't rid of. You can move your warlord in any empty slot and fill rest slots with available creatures or leave it empty at your choice.
Army power is based on total power of your creatures. Synergy doesn't affect AP, but Sets do. You can see your creature AP in its characteristics:
Your total AP affects what happens when you open guild crypts. The quality of your crypts increases if you hit 10,000, 25,000, or 50,000 AP. This bonus is, like your AP used in guild-vs-guild PvP, based on the highest your AP has ever been in any squad.