While T2 is definitely a direct sequel of T1, you have to realize, these are both games that focus on "kill everything that moves" and "loot everything you see". The storyline is only there to help players become familiar with areas and the enemies that arise from them, but isn't necessary play T1 over T2.
Honestly though, T2 is a completely reworked version of T1 that features a whole slew of new improvements, as well as dedicated modding support, etc.
Consider this, it's very much like the L4D series. While L4D had a storyline, and so did L4D2, the story lines were only vaguely connected, but the game was more about a zombie co-op game. This isn't to say the storyline was non-existent, but it's there to guide players to understand what's happening around them. Gameplay-wise, much like TL2, L4D2 is a very very nice upgrade to L4D, in that there are more weapons, more game modes, more special infected, and other nice sweets.
Basically, consider T2 over T1 as a nice upgrade, with more features than its predecessor. If you really really care about storyline, by all means, it doesn't hurt to play T1. If you really like gameplay, then I'd say it really doesn't matter.
Some destructibles that aren't directly targetable do nothing upon being broken, but there are a few that drop loot depending on the setting.
For example, crystals jutting out of the ground in caverns and mines can drop ember shards, which are hugely valuable for upgrading weapons and gear. However, most lanterns and other light-emitting objects give nothing.
Also, any destructible you hit continues building charge. So can destroy them while you travel around to maintain charge.
Best Answer
According to this wiki, all pets are equal.
However, different fish produce different properties when fed to your pet.