Hi, cats can be very territorial and it seems like it may be a case of your resident cat is unaccepting of the foster cats. There are some tips for introduction and cohabiting for cats but with the reaction you have described I would suggest not fostering until you have a cat free house. As this will be having a negative effect on your felines welfare
For now I would suggest a product like feliway as this will send 'harmony messages' to your cats and is clinically proven to reduce tension or conflict.