what they are saying is the size(gigs) does not matter, the slot will accept anything in it as long as its the right type of ram chip for the slot (ie ddr4, ddr3, etc)
no you can not really put in different type of ram in a slot it was not meant for, you probably couldn't get it to go in anyways as they all are bigger, smaller, notched differently from ddr2 to ddr3 and so on.
so if your ram chip is of the correct type for the motherboard yes you can put in a 1gb chip into a ram slot that can support up to a 32gb chip. some can be done individually and some has to be paired but thats on the motherboard and its manual will tell you.
hopefully that might answer your question.