Does this prevent impartial fills? I am paying my broker twice or more in commissions when this happens.
Yes. Not all exchanges support this so I suspect your order is routed to unfavorable exchange. Twice in commissions or twice in fees?
Oh, don’t use FOK. You should not be charged for partial fills as long as you don’t cancel and resubmit.
Have you considered using (All or None) ? it is similar to FOK with small difference. With FOK, it is time based, and trade has to be filled immediately, else get canceled. With AON, you order is active until it gets filled/
Doesn't seem like my broker has that unless it's under a different name. The options I have are GTC, OPG, IOC, FOK, GTX, GTD, DAY+ AND GTC+.