I think the choice of the driver is very important for a good synergy. You cannot just come with two iems that use three drivers (in total) and say you want an iem out of it. Like the Custom2 + the HF5 doens't look like such a a good idea.
Look: people who added a driver to their Triple Fi seemed to be happy with result, as the (recessed) midrange was brought forward. So the added driver is probably focusing on the mids (which were more or less the "problem" of the Triples). But if your drivers do not complete each other, it has no sense to put them together.
I would try to take the HF5 driver and add a bass driver to them.