What twtwtw says is correct, you cannot incrementally add the CPU power of a Mac mini to an iMac
What you can do is:
A) As you suggest, use the iMac as a monitor. If for some reason that doesn't work, there is a product called ScreenRecycler that will do this for any Mac.
B) You could use Screen Sharing into the Mac mini from the iMac and run some apps on the iMac and some on the Mac mini, all from the comfort of your iMac.
C) You could get a decent sized monitor for the Mac mini (24" 1080p's are cheap), then use Teleport to control multiple Macs using a single keyboard and mouse
I use this at work between my 27" iMac and my MacBook Pro. In the past I've used it to control 3 Macs. Very handy.
Nope. Not as I suspect you're envisioning things.
The connection between the microprocessor and the microprocessor cache and the RAM memory inside a Mac is vastly — vastly — larger than what's available over a network link, and with vastly — vastly lower — latency than what's available over a network connection, and with larger bandwidth and lower latency than what's available over a Thunderbolt link, too.
Thunderbolt is used as an I/O link in recent Mac systems, and while it's fast, it's not as fast as the memory and cache speeds within the box.
This difference in bandwidth and in latency means that all the data that an application is always tossing around inside itself works as you expect it with the data and the threads all running with a local processor or local multi-core processor like a Core 2 Duo, but applications do not work quite so well (or as fast) over a network connection or a Thunderbolt connection.
This is the difference between the responsiveness you see with a local wired network such as a Gigabit Ethernet network, as compared with an average DSL or cable TV broadband connection, or with dial-up. The local wired Ethernet is far faster. Same sort of thing happens inside the computer, too. That's the bandwidth and latency mentioned above.
There are some computers that do have ways to easily swap processor boards or even to add processors, but that additional hardware is expensive, and those boxes are usually much bigger, and often much uglier. Some of those systems don't have the same speed with the added processors; that the connections are non-uniform. But those connections are still faster than Thunderbolt.
As twtwtw writes, yes, distributed computing is quite possible. This whether you're using Xgrid or another distributed-computing software platform. This all works nicely, too, where applications are designed to use the grid. (Most applications, unfortunately, are not written to use Xgrid or another distributed computing.)
Put another way, you're probably not going to get where you want here, with this hardware.
Sharing display screens can and will work here, but you're not going to find a way to acquire something similar to more microprocessor cores in your Mac.