Good support response, but dll uploads are blocked, making these vanilla only servers. It's impossible to use any mod that requires a dll such as L4DToolz, bebop, left4downtown, sdkhooks, etc. Also, Metamod and Sourcemod can be installed from their Mod manager, but are outdated so this causes problems with any updated plugins. Since they rely on dll's, you can't update them manually.
Response from Streamline Servers:
02 November 2015 at 17:29
No worries, I have reviewed your ticket with us.
I can see you were assisted with and all these requests were added to the control panel within an hour of you contacting us.
We restrict .dlls for security reasons, this falls in line with most other GSPs.