1. The Windows Defender in Win 7 is NOT the same Windows Defender in Win 8/8.1.
In Vista and Win 7, Windows Defender is an anti-spyware. It is considered base-line protection and for some inadequate. Microsoft Security Essentials (MSE) was introduced and at that time considered very good anti-spyware and anti-virus program. There are many 3rd party AVs to choose from too. Many 3rd party AVs and MSE will disable Windows Defender after installation. That renders WD useless.
I had MSE installed on my Win 7 but switched to Avast ( free edition ) upon the news of poor performance reviews of MSE. Now I have both WD an Avast for protection.
In Win 8/8.1, Windows Defender is actually a modified version of MSE.
At the beginning, I also switched to Avast, but decided to come back to use WD ( aka MSE ) as real time protection, and Malwarebytes (free edition ) as on-demand protection. So far no security issues.
I have since 10/01 installed Win 10 TP and I use the same setup as Win 8.1.
Windows defender and Malwarebytes.
2. Speccy is incompatible with Win 10. But it's not the only program incompatible.
Win 10 TP is only in infant stage. That is why we are given a Feedback program so that we can report all the bugs and glitches back to MS. Perhaps, when the Final Release comes out, all dust will settle.
You are not supposed to install Win 10 TP with the intention of using it as primary OS. If you were using this MS link to install, you should have read the Warning note.
http://windows.microsoft.com/en-us/windows/preview-iso