The post above mine did remind me of tanks!
I've kept mice & dwarf hamsters in 10gs before, and they do perfecty fine as long as its cleaned regularly(which I know you would do) so that ammonia doesn't build up. Cats are able to sit ontop if you have a wire hood(like the kind sold at petstores for reptile)... and the weight of the aquarium makes it pretty hard to tip over... none of our three cats have tipped one over. For your two gerbils you could even buy a 20gl(20 gallon long) so that your girls have extra room, and its even less-likely to tip over. You can usually find a used 20gl in good condition for a good price($15-$20) in the classifieds.

Just a suggestion, if you still feel that it will be in their best interest to be rehomed, thats perfectly fine. You know your gerbils way more than I do!
I know this is a super tough decision, and I know you'll do whats best. =)