posts_per_page on category.php and in slider

I love this theme. So clean! But I had a few tweaks I wanted to make, one of which is listing the artwork on the category page in a grid, like this: http://metanoiasacredarts.com/?cat=6
with all the works on the same page (not having to go to “Older Posts” or whatever.)

We have quite a few works to add so there will be about 20 in each category, so I changed the posts_per_page in loop-slider.php and functions.php to 99. Well, that tried to put all the works in the slider.

My question is: is there a way to change this somehow so that it affects the category.php page but not the slider?

Thanks,

Melissa