RIP-1 and IGRP are both classfull routing protocols, in that they send the subnet mask in the routing updates. If the router is incorrectly configured with different masks on different interfaces which mask is assumed?
i.e. Router has interfaces
Serial 0/0 172.29.1.0/24
Serial 0/1 172.29.2.0/24
Ethernet 0 10.1.0.0/16
and recieved RIP-1 updates through all three interfaces, what happens.
What would happen if interfaces had mismatching masks is that each interface believes that it is correct and the other one(s) is wrong. So if it receives advertisement of a subnet of the network that it is in, it will apply its mask to that subnet as it is put into the routing table. And it sends any updates for subnets within that classful network whose mask matches its own mask and drops any updates whose mask is different.
But there are a couple of things in your post that need comment. First your beginning sentence gets it backwards: RIP-1 and IGRP are classful because they do NOT send subnet mask in the routing update. Second since your serial interfaces are in class B network 172.29 and your Ethernet is in class A network 10 there is no mismatch of masks. For there to be a mismatch of masks which causes a problem the interfaces with different mask must be in the same classful network.
So in your example the router may learn various subnets of 10 through the Ethernet and learn various subnets of 172.29 through the serials. And with a classful protocol it will auto summarize and send only 10.0.0.0/8 out the serials and send only 172.19.0.0/16 out the Ethernet.