My gliders hissed at each other from their respective cages, while looking at each other, before I introduced them. I've now introduced them, and Pi in particular is hissing at Miko quite a bit, especially around food so I'm thinking it's agressive but I don't really know.

They are talking to each other. I would not consider hissing as aggressive. I know hissing is aggressive behavior in cats but not gliders.

Now if they are crabbing at each other I would say they are about to get aggressive with each other.