I dont know about the rest, but Refco/FXCM guarantee stop orders. No problems ever with this specific issue. Always got filled on my stops

. There were too much in recent months - the stops, but they were all filled as always.
The price: not the tightest spread out there. 4 pips on EURUSD and 5 pips on the rest. No slippage ever. Customer support = first class.
Although had some problems recently. Couple times didnt get the instant fills that they promise. Happned 3-4 times last month and one time this month. But its not something their dealers do. At leadt I dont belive so. The thing is that after I am set to enter the market (I mostly enter on matker price), and I press the last "ok" button, I got the message "price has moved". This is not exactly requoting, but its almost the same.
The reason why I say I think its not something their dealers do on purpose is that the last time I got that message, I wanted to buy, I got "price has changed", and it actually jumped 5 pips down, so entering on new price was in my benefit... maybe it was still dealers game and their mistake, but this is the situation.
As for guaranteed stops - never had a problem with that. I trade around 3 million tickets, but I asked them once and they say stops are guaranteed no matter the size.
Hope it helps.
Rezo