Let me pick up this one, since the Canon History is the province of Cosmic Starfire, and the Lead Designer, as well as chief cook and bottlewasher for Cosmic.
Let me address a number of points...
First of all, the site that you linked belongs to a friend of mine who has his own, completely unofficial future history for the Alliance races post-Insurrection that includes the events listed on that page. However, it should be understood very clearly that his vision is entirely unofficial and does not represent the direction that I hope to go. None of the new races and new polities mentioned in his unofficial history should be considered official.
Secondly, there is no "Crucian Union". Never has been, never will be. The correct name is "The Star Union". Not the "Crucian Union". Not the "Star Union of Crucis". (The Star Union of Crucis name is the "incorrect" name used by the Federation because they don't understand that just because the Crucians are the de facto leaders of the Star Union, that doesn't mean that the Crucians wanted their racial name appended to the name of their star nation.) The Star Union. The "Crucian Union" is the name my friend insists on using for the Star Union, even when I tell him repeatedly that it is not the correct name. And given that I am the original Creator of the Star Union, I think that I know what its proper name is.
(( Club, please note that that the previous paragraph's tone was NOT directed at you... This entire "Crucian Union" thing is a sore subject for me.
Third, as for the role of the Arachnids and their "World which must be Hidden" in the future of the Canon History, I have no current plans involving them, post-ISW4. That's not to say that they won't be used. Just that I have no plans at this time to do so.
You have to consider this... The Arachnids' single remaining world was probably only a Small population, maybe
likely a Medium population by the time they cut off all contact with it. And even assuming some rather prodigious population growth rates for the Bugs, it's unlikely that they'd be large enough to pose a serious risk to the star nations of the Grand Alliance for a number of centuries. Oh, I suppose that with some extreme growth, these Hidden Bugs could pose a risk to any unsuspecting lesser races they might encounter if they explored away from the space of the Grand Alliance. (And who knows where this "hidden system" may be or where any WP's connected to it may lead to?)
Would the Bugs change their design philosophy? IMHO, yes, they would. Oh, the Hidden World might start with a small-ish defense fleet of ISW4 era designs, but I believe that over time, they would understand the failure of their fleet's designs and would adapt to new, (hopefully) more effective designs. The Bugs may be slow to adapt to things like this. But in this instance, they will have had plenty of time to think about their fleet's design failings.
As to what future Bug designs might look like, I cannot say. I can only say that they will try not to make the same mistakes that they made in ISW4.
EDIT: Let me add that certain future plans I have may make any thoughts of a future Arachnid war with the Grand Alliance ... rather academic. (he says mysteriously...