Different FPS on different clips (premiere pro) so I'm editing a video for YouTube and I'm having some trouble.

I want some clips in the video to play at low FPS (8fps or so), they are recorded at 8fps but, I'm speeding them up (I, however, still want them to play at 8fps)

I did it by setting the project fps to 8fps. The actual problem is, other clips, I want them to play at 30 fps but as the project fps are 8, they cannot. If I set project fps to 30, then the videos recorded at 8fps that I have speed up will play at 15fps, 20fps, depending on how much I speed them up, but I want those to play at 8fps always.

Any help?
What you want to use to have a clip play back at a lower frame rate is the Posterize Time effect. It's pretty straightforward and self-explanatory. Just find the effect in the Effects panel under Video Effects → Time → Posterize Time and apply it to your clip. Then select the clip, open the Effect Controls panel and set the Frame Rate parameter of the effect to 8 (in your case). Then you're pretty much done. 

To apply that effect to multiple clips at once, you can use an adjustment layer. Create a new adjustment layer (File → New → Adjustment Layer) and put it above your clips in the time line. Then apply the Posterize Time effect to the adjustment layer. It will affect all clips in all video layers below the adjustment layer.