1. Probably not: even the Military versions have **** guns. Don't expect civilians to get any better. Possibly flak guns might be available, but I don't see anything better than standard blobs, subachs, and flak. Civilians don't go into battle-zones, so being unable to defend themselves for **** is a given. It'd be unrealistic otherwise, to be honest.
2. Lokis? Herc 2s? No. Maybe decomissioned Apollos, Valks, and Angels. Probably stripped down. Do you see civilians flying F-22's?
3. Maybe Fenris and Aten cruisers. Probably nothing more, and probably stripped down at that (no more anti-cap beams).
4. I'd say limited to AAAfs. I doubt Tritons or Argos have the power grid to fire these even. So...AAAfs on those retired Fenris cruisers. Remember, the Hippocrates in maintained by the GTVA, and probably had a wartime refit. Normally it probably wouldn't have its LTerSlash.
5. Probably armed transports (meaning, argos with Flak Guns at most). Pirates probably wouldn't get their hands on a cruiser very commonly - any cruisers they have would likely be from captain-turned-pirates (which somehow avoided getting hunting down), or something pulled from a scrapyard. Rebels, on the other hand, may have more cruisers due to well, defections. (The NTF even had destroyers, but I doubt you're talking about Rebels on that scale).
I'd think it is impossible to modify a transport into a beam cannon platform, but putting some flak guns in to make it a gunboat would be feasible. Do you see any oil tankers with battleship turrets?