Do you really need an external clock? AFAIK the Attiny841 can use its internal clock to generate 8MHz
Is the pro mini using a resonator or a oscillator.
What makes you suspect the problem is the clock source? Problems with internal clock come from it not being accurate enough, and typically impact only serial and other timing-sensitive things (like outputting pwm at an exact frequency) - and even so, the attiny841 at 3.3v is calibrated to within 2%, making it fine for serial. Triple-check all wiring (make sure you're not getting it mixed up with the pinout of the ATTiny84). Also, you often need additional capacitors for decoupling/filtering with the NRF24's - many of the modules don't have sufficient decoupling and when they try to transmit, the current spike puts a glitch on their power rail sufficient to hang the NRF24. A quick bit of googling will get suggestions for the capacitor values. You did do burn bootloader first, so it's running at 8MHz not 1MHz?
The el-cheapo Pro Minis mostly use a reasonator.
I was looking at a HC49 8mhz crystal but it takes up a lot of space on the board, almost 2/3 of the width of my board. I was wondering if a oscillator or resonator (in smd form) would work. How do I which one to go with.Thanks!