Creating a bootable clone

I have a new 27" iMac and am interested in creating an external bootable disk, mainly so I can perform certain maintenace on my HD, like DiskWarrior that can't repair the start-up disk.

Per the local Apple's store genius' advice and instructions, I bought a 32GB flash drive (a PNY compact model that was on sale). I formatted it, created 2 partitions so to create also the Recovery Partition, all according to the instructions. It took about 4 housr to install the system, which was predictable.

Trying to boot up from that flash drive --- it took about an HOUR to even show the desktop, and even then, my int. HD would not show up mounted, only my patitioned ext. drive.

So first, CAN I AT ALL USE A FLASH DRIVE AS A BOOTABLE DISK?

If not, how can I use my ext. drive for that? It's 1TB size, and already divided to 3 partitioned. MY QUESTIONS ARE:

--- Can I use DiskUtility to CHANGE THE SIZE OF AN EXISTING PARTITION, MAKE IT SMALLER? (There's no need for more than about 20GB for a startup disk)

--- CAN I ERASE/REFORMAT ONE PARTITION, vs. the whole drive?

Thanks so much....