Considered that you have a well maintained, lightly stocked planted tank... a dead guppy somewhere isn't going to cause a problem. Any extra ammonia/ammonium the decay is going to cause over the guppy living is going to be used by either plants or bacteria pretty fast, it's not like it's a 4" goldfish.
If the spike was real water changes must lower it.
You added 2 rams (1" or 2"?) but I think you said you didn't even feed for the first day they were in there? For sake of argument say those 2 rams are as much of a bioload as all the guppies and cories you have in there. The bacteria would have kept up in 12 hours, even if you have the lazy slacking type, just 24 hours to adjust to the bioload.
None of this says that is a real spike.