I'm not trying to be rude, but if I were you I would be much more worried about how you're going to keep that new Malu happy and worry later about an H. Mag.
The best advice I can give you otherwise though is to learn everything you can about the species you want to keep. For example, as was mentioned in your other thread, that Malu is a sand dwelling nem, but Malus prefer to wedge themselves between the rock and the sand. For H. Mags, they are the complete opposite, they prefer island type structures and you're looking at at least 40 gallons if not honestly probably closer to 75 without prior experience.
Learning the little nuances about a particular species before you buy it is going to lead to success, the best way to learn this IMO, is scour as many forums you can relating to the topic and find what works for people and what doesn't.