If RIPV1 can only populate its routing table with the above info,
any network with a classless mask simply wont matter, it wont make it into the routing table.
Yes, Unless it's a directly connected network, then the router will use the subnet mask that is already available on the interface it self. And when RIPv1 send broadcasts without subnet masks (comming from that same interface from the neighbouring router) telling about remote neworks, Then if the remote network has the same classfull network address as the one for this interface, The routing table will use the same classless subnet mask of the directly connected network for that remote network.
Sorry if that does'nt make sence but I tried it with RIPv1 with 3 routers connected with serails. and it works. first link has 10.0.0.0/16, and second link has 10.1.0.0/16. now this is not classfull subnetting and IT WORKED with RIPv1