Q: Cant install OS X on Mac Pro due to SSD replacing.

You will have to do the following:

Snow Leopard Erase and Install

     Be sure you back up your files.

1. Insert Snow Leopard DVD or the installer disc that came with your computer into the optical drive and restart the computer.
2. Immediately upon hearing the chime hold down the C key.
3. Release the key when the Apple logo appears and wait for the loading to finish.
4. After the installer loads select your language and click on the Continue  button. When the menu bar appears select Disk Utility from the Utilities menu.  After Disk Utility loads select the out-dented disk entry from the side list (mfgr.'s ID and drive size.)  Click on the Partition tab in the Disk Utility’s right window.  Set the number of  partitions to (1) from the drop down menu. Click on Options button and select GUID, click on OK, set the format type to MacOS Extended, Journaled. Finally, click on the Apply button.
5. After formatting has finished quit Disk Utility.  Continue with the OS X installation and follow the directions.
6. When the installation has finished the computer will restart into the Setup Assistant. After you complete the Setup Assistant you will be running a fresh installation of Snow Leopard. Open Software Update and install the recommended updates.
7. If you are planning to sell or give your computer away, then do the following: After you reformat your hard drive and reinstall Snow Leopard, the computer restarts to a Welcome screen. To leave the Mac in an OOTB (Out Of The Box) state, don't continue with the setup. Instead, press Command and Q keys to shut down the computer. The Setup Assistant will guide the new owner through the setup process after he/she turns on the computer.
8. Download and install Mac OS X 10.6.8 Update Combo v1.1. if needed. It is not needed if you are selling or giving away the computer.

What version of OS X came with the computer when it was new? If it came with Lion pre-installed, then you should be able to do the following:

Install Lion Through Yosemite on a New HDD/SDD

     Be sure you backup your files.

1. Restart the computer and after the chime press and hold down the COMMAND-OPTION- R keys until a globe appears. Wait patiently - 5-15 minutes - until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu and click on the Continue button.
3. After Disk Utility loads select the drive (usually, this is the out-dented entry with the mfgr.'s ID and size) from the side list. Click on the Partition tab in Disk Utility’s main window.
4. Under the Volume Scheme heading set the number of partitions to one volume. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the Format type to Mac OS Extended (Journaled.) Click on the Partition button and wait for the process to finish. Quit Disk Utility and return to the Utility Menu.
5. Select Reinstall OS X from the Utility Menu and click on the Install button.

If you boot with the Option key depressed and the El Capitan installer connected can you boot from the EL Capitan Installer?

If yes the installer includes Disk Utility. What does Disk Utility show about the HD? Is it formatted in Mac OS extended (journaled)?

If the HD does not show you may have a bad HD or a defective flex cable between the logic board and HD. The flex cables are easy to damage.

Sounds like your model came with Snow Leopard 10.6.4 pre-installed meaning it cannot do a Network Install. You would need to find the software discs that came with your computer when it was new. if you don't have them or lost them, then you can call Apple Support and purchase replacements. Or you can carry it down to your local Apple dealer and see it they can install it for you. If you made backups you can restore from a backup.

I don't know what else you can do with an erased HDD and a new, unformatted SSD that is also empty.

How was the bootable installer made?

You just do not copy the download from Mac App Store to the USB media.

You can make it using Terminal on a Mac.

You can also use this app on a Mac to make a bootable installer

http://liondiskmaker.com/

I don't know unless the system on the USB is faulty. Insert the USB disk into one of your USB ports. Restart the computer. At the chime press and hold down the OPTION key until the boot manager screen appears. Select the icon for the USB disk then click on the Arrow button below. The computer should boot from the USB disk unless it is corrupt or has a non-working system on it.

If the installer does not show when booting the the Option key depressed then it is not bootable.

What does show when you boot with the Option key depressed?