No Alfa 155 was not their competitor, Alfa 75 was, and Ford Sierra RS500, Vauxhall Cavalier, Audi quattro V8 and Ford Mustang for example. There is no Audi sadly, but here are the skins for BMW, Sierra and Alfa 75 (also links for Sierra and Alfa cars in readme). This three were running in WTCC 1987 together. It is an excellent championship when you put them together and can drive all three of them, they are all fun. I guess you could put 190e in there aswell. Not historically corect, but it's there, there spec wise:
https://www.racedepartment.com/downloads/1987-wtcc-skinpack.19926/
Here is my BOP if you want:
https://acstuff.ru/s/shY5
And Cavalier was made by Shaun Clarke both 1990, which is relevant here, and also 1995, which is not. Both are great cars:
https://shaunclarke35.wixsite.com/website/downloads
If you have the Simdream legends packs, here is my BOPs for BTCC 1991 season, where you can drive with the bmw, cavalier or Sierra and use Simdream cars for filling up the field. Skins for BMW are also available on Race department:
My BOP:
https://acstuff.ru/s/nmL6
For early 90s DTM and both Mercedes and BMW, there is no Audi sadly, so anything you put together will be incomplete. There is also no Mustang and no Opel Omega. Other cars that are available are i guess Miitsubishi Starion Race and some others as well. Search for Group A cars from late 80s.