DC9 FLC LND ON MEM RWY 18L; ARE CLRED TO CROSS RWY 18C. WHILE CROSSING RWY 18C; THEY ARE OVERFLOWN BY A SINGLE ENG ACFT.

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LAST LEG OF 3 PAX TRIP. TWR TOLD US TO EXIT AT THE HIGH SPEED; CROSS RWY 18C AND CONTACT GND ON THE OTHER SIDE. WE DID. BEFORE XING 18C; WE CHKED THE RWY VISUALLY IN BOTH DIRECTIONS AND WE BOTH NOTED AN ACR HOLDING IN POS ON 18C. AS WE CONTINUED ACROSS (PER TWR INSTRUCTIONS); A GENERAL AVIATION ACFT PASSED OVER HEAD AT BTWN 100 TO 150 FT AND CLBING. APPARENTLY; HE HAD JUST LIFTED OFF OF RWY 18C. NEITHER PLT SAW HIM BEFORE XING 18C BECAUSE WE MADE SURE THE RWY WAS CLR; NOT THE AIR SPACE ABOVE IT. THE WINDOWS RESTRICT VISIBILITY UPWARD. AND WE ASSUMED THE ACR IN POSITION WAS THE ONLY ACFT USING THE RWY. ATC IN MEMPHIS IS A LITTLE LOOSER THAN IN MANY OTHER FACILITIES. THERE HAS ALWAYS BEEN A BIT OF A COWBOY MENTALITY. THE SAME CTLR WAS CTLING BOTH RWYS. HE WAS HANDLING ALL THE PLAYERS AND STILL; IN MY OPINION; LOST SEPARATION. IF THE GA DEP HAD LOST POWER; HE MAY HAVE BEEN ON TOP OF US. IT WOULDN'T HAVE BEEN HIS FAULT. GA PLTS SHOULD ALSO BEEN ENCOURAGED TO USE THEN LNDG LIGHTS ON TKOF AND APCH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR'S OTHER CONCERN IS THE FREQUENT; CLOSE-IN RWY CHANGES THAT THE FLC PERCEIVES AS THE CTLRS 'DOING YOU A FAVOR' WHEN ATC ASKS FOR A GATE NUMBER. THE CAPT STATES THAT THIS CLOSE-IN RWY CHANGE CAUSES UNNECESSARY COCKPIT WORKLOAD; THAT IT WOULD BE ADVANTAGEOUS IF THE RWY ASSIGNMENT WAS ESTABLISHED MORE THAN 20 OUT TO ENSURE AN APCH BRIEFING COULD BE COMPLETED; AND BEING DELAYED VECTORED FOR THE 'BETTER RWY' CAUSES MORE DELAY AND FUEL CONSUMPTION THAN LNDG AND TAXIING TO THE GATE. THE FLT CREWS CONCERNS HAVE BEEN FORWARDED TO COMPANY AND TO THE TWR IN THE RECENT PAST WITH NOT APPARENT CHANGE TO THE TWRS PROCS.