Thanks for the tip Tigersim - that may help quite a bit.
I found they were less likely to disappear if I did not let my sims travel, although I may just have gotten far enough ahead of the sims disappearing. Mine were disappearing literally overnight, after my sim had made friends with them. VERY frustrating.
For challenge (and dynasty) purposes, it may be worthwhile to get the mixology career out of the way early. Depending on the rulesets, the Mischief aspiration might be another early candidate, as there was only 1 inhabited household left in my town by the time gen2 reached elder (1 youth potion for gen 1 YA, and 1 youth potion for gen2 child), and that house I had populated myself from the exchange! You need to visit 5 homes for this aspiration and clog their drains.