I don't really have any advice I'm afraid.. just wanted to say that Sophie and Mikah got off to a bad start (with Sophie being a new/young mum etc) so reintrocing *might* work... but I would guess a slim chance

I know Cats Protection and several other rescues in UK prefer that you don't intergrate foster cats with your pets as it can be emotionally upsetting for the resident cat to bond then have their playmate removed as they go on to a new home, once or twice doable but on a regular basis is very upsetting and stressful.

Personally in your situation I would probably put Mikah first and either get a permenant friend or not foster if you don't have the room to always keep comfortabley separated