Technically, yes, you can mix the RAM; it's just not recommended. Consdiering that you're using such old hardware, I'd assume you're not all that concerned with performance.
When you mix RAM, your RAM performance will be limited to the "weakest" module. By "weakest" I mean the one with the lowest frequency (MHz); the highest latency; and higher voltage operation (this is because it would need more power).
As far as the 1GB module not being recognized, there could be several reasons:
1. Defective module
2. Defective DIMM
3. Unsupported module (not all RAM works on all mobo's)
Download MemTest86+ 4.1
and burn the .iso to cd. Restart your computer and then let the test run itself. Allow the test to run through 8-10 "passes" (cycles), this will take several hours.