As Rich has said iMovie version 9 ( aka iMovie '11, included in ilife '11) does not perform well in El Capitan.
iMovie version 8 ( aka iMovie '09, included in iLife '09) is very similar to version 9 and would therefore most likely suffer in the same manner under El Capitan.
If you want to try iMovie version 8 to see what complications may exist then you can probably install this into El Capitan if you still have the 2 grey install discs that came with your 2009 machine.
One disc is the OSX install and the second additional software such as iMovie ,iDVD and other stuff.
You can do a custom install that allows you to just install iMovie if you wish, look out for the Custom Install
button bottom left of the install window.
If you are able to obtain iMovie, just be prepared for glitches as Rich has indicated .
Update to version 8.0.6 here
iMovie 8.0.6
This is an update so you need to install the original from the disc first.
If you do have the 2 install discs, Snow Leopard I assume, then you could install Snow Leopard on a separate partition on the SSD.
If you have installed the SSD yourself then you will know what you are doing.
If you need additional info on creating partitions on a Mac then there is plenty of info on the internet.
I think the best way of doing this is to obtain a clone of your El Capitan system on to an empty
spare hard drive.
Then create 2 partitions on your SSD ,or more if you want, but 2 will do.
Install Snow Leopard on one partition from the install DVD and copy back the clone of El Capitan to the other partition.
iMovie version 8 will run just fine on Snow Leopard.Install it as mentioned above.
If you don't have too much on the SSD at the moment then you can clone to a USB stick if large enough instead of an external drive.
Use Disk Utility to Clone a Mac's Drive
A bit of research may be required but you end up with both Snow Leopard and El Capitan on your MacBook.