Yes. HD doesn't matter, just size and creature type.
The reason why there aren't any restrictions is that all the benefits are carefully rationed.
The reason why there aren't any restrictions is that all the benefits are carefully rationed.
Well, certain forms might give better speed or base natural attack damage than a very similar form, for example.
Important to remember that you keep your druid's ability scores in the new form, you do not inherit the stats of the target creature. The size aspect is handled by direct bonuses and penalties as described by the spell.
So as a Dire Tiger, you'd only have your strength + 4, not the listed strength of 27. This will mean the attack and damage numbers listed on the stat block of the Dire Tiger will also change to reflect your strength, not the tiger's.
The reason to go with the tiger is the number/types of attacks, low-light vision, scent, grab with bite attack, and the ability to pounce.
Check out http://www.d20pfsrd.com/magic#TOC-Transmutation-Polymorph for exactly what you get when transformed into another creature.