Okay, answering myself

(1) It is possible to install both eCS and Debian Squeeze on the same computer! Here is one such computer

(2) GRUB 2 is not guilty for problems with installation! GRUB 2 is
okay. Install GRUB 2 on the root partition.
(3) The one who causes problems is Debian fdisk. But this is nothing new: make a partition with Lenny fdisk and LVM will stop talking with you.
(4)
Trap! Debian Squeeze fdisk numbers partitions from 1 and partition #1 is the
last partition on the disk. When installing, note carefully which partition is your root!