I did new tests. For the first question i.e. installing firebird, I indeed don't need compat-gcc.
I tried :
yum install compat-libstdc++-33
rpm -ivh FirebirdSS-1.5.3.4870.nptl.i686.rpm
or
rpm -ivh FirebirdSS-2.0.0.12484-ReleaseCandidate1.nptl.i686.rpm
Firebird installed and started fine.
I think, compat-gcc was necessary on SME6.0.
For the last question, when
yum --enablerepo=base install compat-gcc-32
The answer is
=============================================================================
Package Arch Version Repository Size
=============================================================================
Installing:
compat-gcc-32 i386 3.2.3-47.3 base 3.0 M
Installing for dependencies:
glibc-devel i386 2.3.4-2.19 base 1.9 M
glibc-headers i386 2.3.4-2.19 base 579 k
glibc-kernheaders i386 2.4-9.1.98.EL base 693 k
Updating for dependencies:
glibc i686 2.3.4-2.19 base 5.0 M
glibc-common i386 2.3.4-2.19 base 16 M
So, I don't know why but it want glibc-devel, glibc-headers, and glibc-kernheaders.
But I can see an other problem. When I want to install a package from CentOS, yum want to update some packages comming with SME. This may not be very safe.