Assuming that you have done calculations on the services and have found that they all are at 150 amps. 5 residential services can be calculated at .45 for a total of 337.5 amps (see 220.84).
6 residential services can be calculated at .44 for a total of 396 amps. Same code reference. Parallel conductor installation of 250 kcmil are good for the 400 amp total (255x2x.8 NEC 310.15(B)(2)(a)) if allowed by your AHJ.
I did not check the nuetral. This also makes the assumption that you meet the multi-residential requirements.
