ストレージ容量はTooling APIを使えば取得できます。
Apexから呼び出すことは可能だと思います。
File storage: https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_limits.htm
Record count per object: https://releasenotes.docs.salesforce.com/en-us/summer17/release-notes/rn_api_rest.htm#rn_api_rest_record_count
https://salesforce.stackexchange.com/questions/193503/how-to-get-storage-usage-organization-information-of-a-salesforce-to-another-s
I think that you can only create one field with the tooling API.