You can have 1 installation of steam. which installs into steam directory. You can have multiple versions of construct installed, even on a usb... The only limitation is what is imposed on the free version.
Scirra installs different directory than steam.
I think you can also install the free version on steam side by side. I think I did this once by accident.
I have a steam license, and I purchased a scirra license. I have steam be the tester, because it is so unstable anyways.