It seems that building assetbundles in the new assetbundle system introduced in Unity 5 is in a way that scanning the entire asset base and the system will build assets modified into assetbundles automatically.
Is there any way to build specific individual assetbundle from assets assigned to it without triggering the scanning of entire asset base ?
If I use the old assetbundle system to build some assetbundles, can they be used together with assetbundles built in new system?
One thing you should know: You need to have an asset that is built into the AssetBundle selected, not the AssetBundle itself. So, for example of you have a scene or prefab that you have included in an AssetBundle, instead of selecting the AssetBundle, you have the scene or prefab selected.
EDIT: Another thing to be careful with: This will falsely overwrite Windows.bundle and Windows.bundle.manifest. So either you make a backup of those files that you restore after building individual AssetBundles, or, when using version control (which you should always do, anyways), simply do a revert / discard changes after building the individual bundles.