Hi daisy2007,
The topper guidelines would be relevant for any type of mattress (memory foam, latex, innerspring, hybrid etc).
Just like with a mattress … the type of material would be a preference choice not a “better/worse” choice so it would depend entirely on which type of material you tend to prefer (either more resilient and faster response materials like latex or to a lesser degree polyfoam or less resilient and slow response materials like memory foam).
Since you purchased a memory foam mattress then that may mean that you prefer the feel of memory foam over other types of materials and if this is the case then I would probably choose a memory foam or gel memory foam topper.
Regardless of your preferences though I would make sure that the topper you choose is a good quality/density and durable material (see the guidelines here) so it won’t soften and break down prematurely although with a topper durability may be a little less important because a topper can easily be replaced without having to replace the mattress underneath it.
I would always keep in mind that you are the only one that can feel what you feel on a mattress/topper combination so if you can’t test the combination in person before a purchase then the return/exchange policy for the topper would probably be a much more important part of your choice so that you have good options after a purchase just in case your topper doesn’t turn out as well as you hoped for when you sleep on it.
Phoenix