iirc, 1.5x2 does not equal flow rate of 3, two 1.5 inch pipes will flow just as easily through a 2.25 or something like that, so 6 tanks all running 1.5 won't require a 9 inch pipe, it'd only need a 5 inch or something along those lines, and I'd think you'd get away with something smaller anyways, as an overflow rarely uses the entire flow capacity of it's pipe....or so my overflow would indicate. I'd presonally use "y" connectors and brach them into to groups of 3 tanks and go with 3 inch pipe at the end so it's twin outputs at the sump into hanging filtersocks or something like that.
Take this with a grain of crushed coral, I've not been doing this long lol
If you have the funds to kick around, you could even go with black spaflex on all 6 tanks and run them all into the sump. 6 - 1.5 inch spaflex hoses would consolidate nicely with the liberal use of brackets and zip-ties, and spaflex makes it easy to sort the hoses into the sump, utilizing space to best advantage.