Clustered vs. non-clustered shouldn't matter for NAT configuration unless possibly you have NAT templates (fwaccel stat) enabled.
NAT determination is made at the start of the connection (receipt of the TCP SYN packet) right after an Accept action by the Firewall/Network policy layer, and cannot change for the life of that connection, even if the NAT configuration is changed and policy installed. This can be particularly vexing for continuous pings that are started, left running, then have the relevant NAT configuration changed. Once the NAT has been changed, a ping running between the same source and destination IPs must be stopped for at least 30 seconds (ICMP virtual session timeout) for that virtual connection to "let go" of the previously determined NAT address and apply a new one. Obviously pinging a different destination IP for which there is no virtual ICMP connection will cause the new NAT setup to be applied immediately.
Bottom line is make sure that a new connection is really starting to properly apply the new NAT, web browsers in particular can be very bad about this with speculative downloading and persistent connections.
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