Atomizer, this is a very trival cfg to do and is done in the same shape & fashion as the cisco ASA to some degree
You will need to enable SNAT on the FGT fwpolicies for tunnels leading from main to cust1 and cust2
You craft your src_subnet for the IPSEC-phase2 proposal, to be the SRC of the SNAT, likewise the cust1 & cust2 will have that address as their DST in their proposals.
Everything else after this is just basic fwpolicy(s) , static routes to the remote ends etc........
So to answer your question, yes the config you have proposed will work. You can source that SNAT with any reachable ip_address that you present in the static-remote-routes and by fwpolicy for the services that you allow.
fwiw
I commonly deploy what your suggestioning , when interfacing to remote vendors and you don' t want to worry about ip_address overlaps that you commonly see in rfc1918 address. Each one of my vendors see a real public address that ' s not overlapped with any of his other customers.