... Outpost "supply lines" should grow Slower than "colony supply lines"
Since building outposts would be the only non-global way to extend supply lines besides waiting, and i think we really need a way to "build" supply lines between colonies, i wouldn't want to see supply line growth handicapped. At least not without an alternative method of extending supply lines (which may be a good idea, but isn't on topic here.
If I place a colony, I have to wait X turns to get a supply linkage, If an Outpost gives a Linkage faster then
If you can place an Outpost on a Conlony: Oh I just Colonized a planet, I want a Supply linkage right now...so I'll put an Outpost on as well)..bad
If you can't place an Outpost on a Colony: Oh I just Colonized a planet, I should have put an Outpost there first.. maybe if I give away the planet, and then bomb it out I can get the linkage up faster by recolonizing with an outpost first)..VERY VERY VERY BAD!!!
As for Building Supply line growth between Colonies, how about a Building... that gives a Linkage>1 (2,3,4, etc.) ie it give a +20,40,60 to max AND Current "Construction" meter. It should fit for buildings, you only need one every 6-10 systems or so. (colonies naturally get a Linkage of 1, once they are at 'construction'=20 if you want anything greater, then you Build a "Starport".)
If a colony has to wait then an Outpost should not be faster.
I do Somewhat Like the idea of different Types of Outposts.
Listening Outpost... good Info gathering capability, no supply linkage
Supply Outpost...minimal Info gathering capability, Supply Linkage (takes same time as a colony)
Research/Mining, etc. Outpost...other specific benefits, but no Supply Linkage
And I see no reason to avoid "outpost spamming" that does not also apply to "colony spamming" (colonies require food, but they can be focused on food)