Adding second firewall to ISP connection with multiple subnets?

I think your best bet is going to be to contact your ISP and clarify exactly what they are giving you with the 203.0.113.88/29 block. There is no reason for things to be complicated by the uncertainty about these IP addresses.

The most ideal scenario is for you to connect a second firewall to that switch and give it one of the IPs on the 203.0.113.88/29 network with a default gateway on the same network.