I have been observing your posts for some time, blutoonwithcarrotandnail, and I believe I have diagnosed your problem with Latin: you are thinking about it too much. Please, spare yourself the agony of grappling with these slippery grammar concepts, find yourself a copy of Lingua Latina or anything that you can read fluidly at your level, and absorb and enjoy the language without getting tangled in the grammatical thornbush you are currently thrashing about in.
To answer your question: mille is used both as an adjective and as an (indeclinable) noun. Mille viri: a thousand men. Mille habeo: I have a thousand. Milia, as the plural of mille, is only a noun, and declines: cum octo milibus pedum, with eight thousands of foot-soldiers.