I used to use AMT a whole lot, way too much, and I would always plug it. At first I would convert to HCl in liquid and plug that way, then I got lazy and started just suspending freebase in water and plugging, and then I got really lazy and would just put freebase dry in a gel cap and insert the gel cap. They all worked fine and the raw freebase really didn't burn much more than any other way, surprisingly. Though I would recommend if plugging a freebase to convert it to a salt first because it's gotta absorb better and I have to believe it's better for your rectal membranes.
But yeah anyway, citric acid is perfectly fine to convert freebase to salt, it doesn't have to be a strong acid... in the presence of an acid, the freebase binds to molecules of that acid to form a salt. You could use citric acid, vinegar, HCl... best to add dropwise until it all dissolves, which for 5-MeO-DMT would probably take one drop (or a few citric acid crystals), since it's such a small amount. I remember before I realized you could use other acids, I used muriatic acid with AMT, which is I think 5% HCl, and it would take about 2 drops to get enough to convert it all to AMT HCl.