Proving grounds endless mode (DPS)

Are the trash spawns fixed or are they just random adds spawning for 30 waves?
The first 10 waves of endless are different to gold, but they are fixed. After that, each 10 waves repeats. Starting from the first wave, all of the enemies have a buff that increases their health by 1%. This buff stacks by 1% every wave, so although the wave order repeats, each iteration of that wave is 10% more difficult (e.g. wave 11 is the same enemies at the same time as wave 1, but is 10% more difficult).

Are there any new adds that didn't appear in Bronze/Silver/Gold?
There aren't any additional enemies in endless.

How hard is it compared to gold?
It's obviously harder and takes much longer (30 minutes approximately), so it will likely be a significant time sink whilst learning. You've already learnt all the different types of enemies in the previous challenges, so you know how to handle them. It is mostly about learning the 10 waves and optimizing your strategy for each wave (plan when to use cool-downs). After that, rinse and repeat, bearing in mind that the enemies will have more health each round.

Is there anything else I get from it besides the achievements?
Proving Yourself: Endless Damage (Wave 30) earns you the title, The Proven Assailant. Your progress in endless is tracked and you can compete and compare yourself with other players. Other than that, no.

Endless Healer (Wave 30) and Endless Tank (Wave 30) also earn a title, The Proven Healer and The Proven Defender respectively.

Can I use raid pots in there?
Flasks and food can be used, but potions and other consumables can't. They do not suffer from the stat reduction, much like enchants, gems and profession bonuses.

A really great guide can be found here. Good luck!