Tom Kalbfus said:
Its not actually possible to guard an interstellar border, Space is vast, too vast for a planet with an average population rating of 5 to actually guard, as there would be enough people on enough ships to prevent a fleet of space gypsies from suddenly emerging from Jump Space at the 100 diameter limit. If the least capable ship had a jump rating of two parsecs, there are a lot of places that fleet could make the jump from. I think the best that may be accomplished is that the local planetary security force will deter the fleet from getting too close. So a Gypsie fleet could be treated as a mobile mainworld, that is it would have a population rating, a government rating, a law level, and a tech level, its size would be 0 as it can be treated as asteroids with engines, the atmosphere on the outside of the starships is a vacuum, so it gets a rating of 0, and of course most starships don't have liquid water on their outsides, so the hydrographic digit is 0 Of course such ships would have water tanks and internal atmospheres. A fleet might even have a mobile starport for smaller ships to land in. What do you think of this idea?
The Fifth Frontier War game shows that there are choke points when crossing areas. And you can't afford to leave a strong planet in your rear because the attackers are even more vulnerable with long supply lines (and those ships have to come from established bases). But essentially you are right, you can't stop somebody intent on jumping in to a system. Which is why battles are almost never fought in deep space and only around something worth fighting over.
As far as a mobile 'world', there's nothing in the rules that say's Nyet to the idea. If you wanted to make it part of your adventure that works. The only issue would be a backstory - assuming you or your players wanted or needed one. It's easy enough to just say "this is how it is" and not worry about it. But if you like background that makes sense then you'll need to figure out just why they travel, and how they pay for it.
The suggestion of using an asteroid for a mobile base makes sense. If they are gypsies (and assuming they didn't hit a lode of minerals in mining out their asteroid base) then they'll probably be getting by on the margin. Perhaps they could have a route of sorts they take, and every few years or so they pass through one or more large habitation systems (and of sufficient tech level) where they have a carnival or festival of sorts for months. And during these festivals they make the bulk of their funds to pay for the times when they are wandering. If they have a colony of artists and artisans, during the slow times they could be making up their wares and such to sell as they pass through these other systems. In between they may have other traders or even their own folk who trade from the base to other systems nearby in their trading ships. They would need to spend a while in a system to refuel such a behemoth, which would give justification for them stooging around a system for months on end.