Minimum Radius Curve-6axle AC4400's

I have some experience with 6-axle SD units... They did not run well on 18" radius... Ran much better on 22". That's the minimum I would recommend.

I am in process of rebuilding my HO layout and have 6 AC4400 & C44-9w's I plan to use on mainline & in and out of a Fuel-Maintenance facility. Can anyone out there who are running the same six axle units recommend the minimum radius on mainline and yards to use for the least trouble free operation??? My room is on average 7.5' wide by 20' long. The wife won't give up any more real estate! I have Walthers code 83 #5,#6,#8 & # 6.5 curve turnouts to use from last layout. Plan to use Atlas flex. Any Help will be GREATLY appreciated! I am trying to get my design worked out on CADRAIL as I appeal for your input. Thanks, Kevin Mahoney