There should be a limit of three, I think, on player ships. It could theoretically be expanded further, but that wouldn't work with the current loadout art. The HUD code should be fine with it (except in small resolutions), i honestly don't know why Volition hardcoded it in the first place, it was so much cleaner to make it infinitely expandable. Secondaries should be the same, too.
So, current limits stand at 3/4 for primaries/secondaries, but I think there might be some kind of limit so that the player can only use 3/3.
You will need the file from the mediaVP (mv_core) or a different background for the HUD to look right though.