You aren't going to be able to get any love from JTAC with a non-Juniper memory stick in the device if you opened a ticket related to memory issues.
Having said that, I have had good luck with Crucial memory (SDRAM) in Juniper routers.
The tricky part is figuring out the specs on the memory although I grabbed an SSG-5 off the shelf and it sure does look like regular old notebook memory.
The socket says DDR STD 2.5V and the module itself doesn''t look like it is anything special.