AlterBrains

If the domains are on the same database will Sh404 sef work ?
that is an essential component for joomla

but then again having 10+ sites on 1 db may not be advisable from a performance or seo standpoint.

Yes, it will work.
No matter if you have 10 sites in one database or 10 databases - performance is same.

Who told you about SEO and databases?! Better change such SEO specialist , SEO is not related to database in any case, no one search engine knows the details of your databases.