Yeah, I've paid attention to all of them in the past, but I personally believe they don't work. At least for me. 50% and full retracement are sort of interesting to me.
set a variable as a flag with value of either 1 or 0 within your condition.
If condition then begin
buy;
ok_flag=1;
end;
then you can refer to this variable to control other parts of your code. If ok_flag = 1 then "DO WHATEVER"
I say that the only Systems that will work are made by traders who actually have experience as to the dynamics that go on throughout the trading day. Basically, if you trade a good system it was created by a good trader.
I vaguely remember one time(during market hours) in the last year it was down for a brief time.
Now, concerning your brokers connection to the market, that is a different story, varying from broker to broker.
Then I guess just go with the execution platform you like best.
Personally, I can only daytrade S&P mini with J-Trader. I use IB for more mechanical and system type trading(API program, etc.).