I never tested memory in a 550; don't have one.
260 MB in the 605/475 was with a 72 pin SIMM doubler and there was no closing the lid. A single 128 MB SIMM is more practical.
Single sided doesn't mean anything with 64 MB SIMMs except in as much as it might correlate with non-composite and/or single vs. double banked. You pretty much always want non-composite though some old composite SIMMs will work (in some machines) and almost all 64 MB SIMMs are single bank -- all RAS wires/signals common amongst the chips on the SIMM.