when you get cassidy as your partner, you have to either find evidence of the vangraffs and mclafferty killing her caravan or just killing gloria van graff or mclafferty... my questions are, after i turn in the evidence the peaceful way is there a way the van graffs wont go hostile? and if i kill them both, will someone take glorias place after a while and sell her energy weapons?

it seems if you do this quest in any way, you wont be able to buy stuff from silver rush anymore

you can buy weapons from then. Get the quest birds of a feather and do that. it gets Cass killed, and they stay friendly.