In terms of most recruitment, go for a type 2 gov where possible. Especially in the Belgic regions. The Milnat and Batacorii you can recruit there are the soldiers I often relied upon to conquer Gaul from Romans in my campaign and push into Italy across the Alps. There I bogged down due to poor immediate recruitment options and the typical waves of Pedites, Triarii, and every mercenary available. In any case, any Celtic region is deserving of a type 2 government.

In Caledonia, I believe you need the 12 turn native MIC for Caledonian Nobles. They are decent medium cavalry, nothing special. They will fulfill your cavalry needs for immediate battles on the mainland but I recommend using Gallic light cavalry or the Belgae variety once you start recruiting in Gaul. Actually shipping the Goidillic cavalry across is a decent option too along with some of the nobles warriors there.

For Siluri Warband, the 9 turn local MIC will work. Not sure why its a local MIC, i suppose its because of their unknown origins? Took me a while to figure that one out considering I resisted checking the recruitment map. Actually had a similar problem with Asturian Axemen as Lusotann. Siluris seem to work well as skirmisher light infantry though you get plenty of skirmisher options as Casse. Love those Balroae!