Hi there! I myself have been experimenting couple weeks with these mods and here's my 2 cents on the subject..
Big car packs seem to all cause the yellow cab issue. According to some forum postings it is because new models are bigger (more detailed) and thus eat up more memory. So when the memory is full. game spawns cars that already are in the memory -> yellow taxi's. I don't know how true this is.
In anycase, there are several scripts out there to make car spawns more sensible, but they all seem to have downsides. I haven't tried these out yet. for example: http://www.gta4-mods.com/script/trafficload-f9780