if i may be permitted to correct a point:
first off, the srd and phb both state in the target line that valid targets are a creature OR one non-magical object of up to 100 cubic feet per level. so it is not creature only.
therefore, the surface in question must be within the size limitations. so changing the ground will not work, unless the dm cooperates with you about granular composition of the ground (for example, everyone standing on a big rock embedded into the dirt or somesuch. likewise, a tiled floor being altered will likely result in only a single tile changing. might be interesting if a vein of mineral was transformed hollowing out the area, change it into an animal and move it out of the mine before it changes back. presto, pure mineral.
so taking your question of surface to acid: the duration will be the result of the total of the duration factors described in the spell. changing solid surface comprised of a mineral into acid (the kind of acid might be important as some plants and animals can produce acid, while there are purely chemical (mineral) acids as well) and assuming that the acid is "mineral" in nature, then you get the same kingdom duration factor of +5, same size bonus of +2, do not get the same class bonus, do not get the related bonus, and get the same or lower int bonus of +2 for a total of +9. 9 or more makes the duration permanent. less then 9 means the duration is limited as per the table.
so you could take the stone surface another is standing on, and if it is somehow a single object within the cubic feet limit (such as a large flat boulder inside the ground) then it could be changed into a pool of acid the same size as the former boulder. that might be an interesting way to steal a giant gold statue... put a portable hole or an enveloping pit at the base of the statue, and then change it into water, or a sheep, or something, and then stick it into the hole/pit and wander off with it.
the spell grants no control over the resulting transformed target.
since the poa spell references the poly spell, i imagine that the limit on HD would transfer as well, so as long as your bearskin dire bear had equal to or less HD as your caster, then it would work.
the HD would serve to incidentally restrict the sizes you can pull off. otherwise, the cubic feet limit would seem to be the only other size restrictive property.
