As far as I know, you should be able to add sda7 to sda6 since they are contiguous to each other. You are using GParted from within Mint, is that correct? If, so, are these partitions mounted when you try this? You can not partition a disk when the partitions are mounted. To simplify this try a GParted live CD or the Parted Magic live CD:
Personally, I prefer Parted Magic; but GParted live CD is fine.
As far as grub is concerned, I assume Mint uses UUIDs to boot partitions in grub and mount them in fstab since that is what Ubuntu uses. The advantage of UUIDs is that if the ordering of your partitions changes (like they will if you eliminate sda7) the UUIDs stay the same, so it *should* have no effect on booting Mint. I assume Mint is sda8. Is that correct?
To identify the UUIDs of each partition run:
in terminal. It will give something like this:
[tom@arch ~]$ blkid
/dev/sda1: UUID="420CB81A0CB80B45" TYPE="ntfs"
/dev/sda2: UUID="11ae0c53-a8d7-464c-aa29-60a828be8ab3" SEC_TYPE="ext2" TYPE="ext3"
/dev/sda3: TYPE="swap" UUID="4533b483-1af3-455e-8ef4-9936467fc292"
/dev/sda5: UUID="54e761dc-3847-4f90-9ea8-5f466c893c0a" SEC_TYPE="ext2" TYPE="ext3"
/dev/sda6: UUID="9d10d416-c356-4b87-907c-2f5e3bc62594" SEC_TYPE="ext2" TYPE="ext3"
/dev/sda7: UUID="1c82c3df-d1de-4d0a-a20c-2321803b4410" SEC_TYPE="ext2" TYPE="ext
If you have trouble booting with grub you can reinstall grub like this:
Sorry I took so long to get back to you on this.