When a creature is surrounded by a swarm, since the swarm is many creatures in a large area, is the creature surrounded flat footed? Can you use a swarm to flat foot a creature? If another creature is threatening along the swarm?
Does a swarm counts as "threatening"?
I think you mean flanked not flatfooted. But in either case the answer is no.
Swarms do not threaten creatures, and do not make attacks of opportunity with their swarm attack. However, they distract foes whose squares they occupy, as described below.
Swarms possess the distraction ability. Spellcasting or concentrating on spells within the area of a swarm requires a caster level check (DC 20 + spell level). Using skills that involve patience and concentration requires a DC 20 Will save.
Also,
Traits: A swarm has no clear front or back and no discernible anatomy, so it is not subject to critical hits or flanking. A swarm made up of Tiny creatures takes half damage from slashing and piercing weapons. A swarm composed of Fine or Diminutive creatures is immune to all weapon damage. Reducing a swarm to 0 hit points or less causes it to break up, though damage taken until that point does not degrade its ability to attack or resist attack. Swarms are never staggered or reduced to a dying state by damage. Also, they cannot be tripped, grappled, or bull rushed, and they cannot grapple an opponent.
do you mean flanking?
couse while you can deal sneak attack to flat footed and flanked targets, these are two different things. flanking some-one normally doesn't make them flat-footed, it does allow to sneak attack him and give a +2 to the hit roll(most of the time, baring things that can't be flanked, uncanny dodge etc)
As LordKalis quoted, “Swarms do not threaten creatures”. Therefore a swarm can’t provide flank.