Just learn that vCenter 6.0 doesn't support SQL Server Express.
The Windows vCenter 6.0 dropped the integrated SQL Express BUT instead it comes with an embedded vPostgres SQL database to replace it with similar limitations. You do not need to install your own SQL server if you're ok with these limits. See this article:
vSphere 6 - Clarifying the misinformation | VMware vSphere Blog - VMware Blogs
Supported databases for the windows installation are SQL 2008 R2, 2012 and 2014, Oracle 11g and 12c as well as the option to use an embedded vPostgres database. vPostgres on windows is limited to 20 hosts and 200 virtual machines. Upgrades where SQL express was installed will be converted to vPostgres. The vCenter Server Appliance supports embedded vPostgres at full scale, 1000 host and 10,000 virtual machines and is the recommended database for the vCenter Server appliance. External Oracle 11g and 12c databases are supported as well for this release, look for these to be phased out in future releases.
It's also possible to keep an existing SQL Express DB when doing an upgrade:
VMware KB: Upgrading to vCenter Server 6.0 without migrating SQL database to vPostgres
If you use your own SQL server go with a 64 bit version and stick to the VMware Product Interoperability Matrixes.