ACR X UNAUTH SPD REDUCTION USED TCASII FOR SPACING. PLTDEV.

Narrative

AFTER CONTACTING REGIONAL APCH ATC INSTRUCTED ACR X TO TURN TO A 280 DEG HDG AT THE 5 DME/296 DEG FIX OFF OF SCURRY VOR. ATC TOLD US TO DSND FROM 11000 TO 5000. JUST S OF NAVY DALLAS; CAPT AND I WERE CONCERNED ABOUT LACK OF SEQUENCING INSTRUCTIONS FOR RWY 35R. THEREFORE; I STARTED TO SLOW FROM 250 KIAS. WHILE SLOWING THROUGH 210 KIAS; ATC INSTRUCTED US TO SLOW TO 190 KIAS AND THEN DSND TO 3000. ATC INSTRUCTED US TO TURN R TO 310 DEGS AND LOOK FOR AN LGT ON A HIGH R BASE DSNDING OUT OF 4500 MSL. WE TOLD ATC WE HAD THE ACFT BUT QUESTIONED OUR SEPARATION AS HE WAS APPROX 3 MI OUT ON TCASII. WE WERE CLRED FOR THE VISUAL TO RWY 35R BUT TOLD TO MAINTAIN 170 KIAS UNTIL THE OM. AFTER ENCOUNTERING SIGNIFICANT WAKE TURB; CAPT AND I DECIDED TO REDUCE TO 150 KIAS. APCH CTL WAS SO BUSY; WE WERE UNABLE TO BREAK IN AND INFORM ATC OF OUR SPD REDUCTION. SHORTLY THEREAFTER; ATC (APCH CTL) QUESTIONED OUR AIRSPD. WE STATED WE HAD SLOWED TO FINAL APCH SPD. THE FINAL MONITOR CAME ON AND ASKED 'WHAT SPD IS THAT' AND WE REPLIED 150 KIAS; AND HAD SLOWED AS WE WERE FOLLOWING AN LGT. APCH CTL SNAPPED BACK AND SAID 'YOU ONLY NEEDED 3 MI SEPARATION.' DURING THIS BRIEF CONVERSATION WE HAD PASSED THE OM. UPON LNDG THE LGT HAD ONLY CLRED WHEN WE WERE LESS THAN 200 FT AGL. HAD WE NOT REDUCED OUR AIRSPD; A GAR WOULD HAVE OCCURRED! THIS PROB AROSE FROM POOR ACFT SEQUENCING; ALONG WITH THE FACT THAT THE LGT OVERSHOT THE FINAL TO RWY 35R; AND THEN SLOWED; AFFECTING OUR OP. THIS; COMBINED WITH AN OVERCROWDED APCH CTL; LED TO THIS SEQUENCING PROB.