Does everyone need to be the same platform on duo trio and squad categories?

no bc spider is a moblie player and he was in a sqaud with pc players

No you don't, But good question i would reccommend putting the platform most people are on, if each person in the run has a different pov i would reccommend the person thats submitting to put their pov on it

Ok ty

do i need to join the discord server to join the 'pro' speedrunners?

technically yes Nou6813 because most of the speedrunners hang out on the discord