Thanks for providing some info for the database. But still i have some questions why some tables have no clustered Index ? And high logical reads with clustered index scan . Is it possible to create/alter index on the existing tables based upon the execution plans .
Please see KB73614 for our stance on custom indexes. In a nutshell you're welcome to add your own indexes, but we can't help you with troubleshooting them and you're responsible for maintaining them.