Currently we have SharePoint 2010 environment, heavily customized (farm solutions), non-multi tenant environment.

We plan to build a SharePoint 2013 multi-tenant environment and migrate from SP 2010 environment to this. We plan to build a multi tenant environment as we have are going to host sites for various customers. 
The customers are in same domain but are under separte OUs in AD.

I have few questions:

1. We have Project server implemented in our current SP 2010 environment. Are they are known issues in migrating it to a multi-tenant environment in SP 2013?

2. Currently in SP 2010 environment we have two MySite hosts (two web applications). How can they be migrated to multi-tenant environment if I plan for a single web app model in MT environment?

3. For farm solutions, I am thinking of as of now migrating them as it is and use in Tenant sites via feature packs. This should be possible. Right? Later on we do plan to move away from farm solutions. 

Hi,

If you plan to build a complete new SharePoint 2013 multi-tenant environment, I suggest you first upgrade the current SharePoint 2010 environment to a normal SharePoint 2013 environment, and then use Backup-SPSite/Restore-SPSite to move the site collections to the new multi-tenants environmet. During this, the path of the site collections might be changed. Consider that you may want to use HNSC instead of the path-based site collections.

 1. The Project server 2010 cannot run under SharePoint 2013. After upgrade, you will need to install Project server 2013.
 https://social.technet.microsoft.com/Forums/sharepoint/en-US/9f7f3318-f3e5-4f1e-bdaa-f4a34367d90f/project-server-2010-under-sharepoint-2013-is-it-possible?forum=sharepointadmin
 
 2. If you would like to consolidate two content database to a single content database, my suggestion is to download all the existing content of My Sites, create complete new my sites in the subscription environment, and upload the content. Before downloading, you can measure the current size of my sites with PowerShell first, because a lot of my sites may be not in use. Please refer to the following link for this http://www.sharepoint13.org/?p=261
 
 3. Correct. You can convert the existing site collections to tenant sites with PowerShell. See http://tutorial.programming4.us/windows_server/Multi-Tenancy-in-SharePoint-2013-(part-1)---Managing-Service-Application-Groups,-Creating-a-Site-Subscription-.aspx

Since the upgrade and migration is a complex change, I recommend you to consider more aspects like the data storage, SharePoint limitation, DNS configuration(for HNSC sites), and the structure for service applications. At last choose the best approach for the change.

Here are more additional information might be useful for you:
Upgrade overview https://technet.microsoft.com/en-us/library/cc262483.aspx
Understanding multi-tenancy in SharePoint Server 2013
https://technet.microsoft.com/en-us/library/dn659286.aspx

Thanks,
Reken Liu