1MB and 4MB 30 pin SIMMs are still relatively easy to find cheaply. 16MB SIMMs are pricey but can still be found. Google for more information about RAM speed -- SIMMs from a Plus or SE may be too slow. 8MB RAM total is great for System 6, but for anything else you ideally need 16MB or more.
And save yourself a lot of time by fitting a NuBus video card. The built-in video connected to a suitable monitor is adequate for a file server, but it is slow in itself and also makes the Mac slower. There are loads of threads about IIci built-in video here that discuss the pain.
Except in the Compacts, 30-pin simms ordinarily must be installed in sets of four. The IIci is like this: you therefore need four identical simms. It will not work with only one.