My standard practice for quite a while has been to use all companions (and some family/clan members) for caravans; a nice way of keeping a steady income if you're not constantly warring or seeking out the best bandit parties/quests. (Though I'm now early in a v1 game where I want to avoid it, to keep more companions for banner/formation/new clan purposes - i.e. them being more part of my kingdom rather than just money makers).
Good guide - the only points I'd add is that I think scouting becomes an decent attribute with early speed perks for a caravan evading bandit parties - and that ultimately, absolutely any companion will be profitable (at least before you're in a kingdom and subject to more attacks). The first couple of levelling-ups for a less-suited companion are a bit of an impatient wait, but once you can add focus to trade (and then scouting/athletics/riding as needed) and an attribute point or two, trade skill increases fairly rapidly for any companion leading a caravan.
In my last game, in the 148 days between clan tier 2 and clan tier 3, I'd got 5 companions working caravans - a spicevendor, an 'ill-starred', an Empire wanderer, a willowbark and a smith. The four non-spice vendors all had reached a trade skill in the range 44-55 at the end of that period and as a snapshot on the day I reached T3 were bringing in 318, 498, 502 and 520 gold. At that stage (pre-kingdom, no wars), none of them had ever been captured. If that daily figure gradually increased from a base of about 150 average to 450 or so, those caravans have paid for themselves about two and a half times over, despite their initial unsuitability.
In the next 350 days to reach T4, those same four non-spice vendors had reached trade skills of 64 (the smith), 95, 101 and 104. Again as a snaphot on that day, they earned 408, 480, 955 (the smith!) and 1251. I think one had been captured in that time - even if it were two, those four non-specialist traders had each raked in perhaps a couple of hundred thousand profit in total.
(By T4, their scouting skills - including the spicevendor - were 55, 56, 64, 64 and 85. If I'm right that the first few perks of scouting are useful for evading capture, all good. If I'm not, I could have focused more on trade alone, increasing those skills still higher, for even greater profits).
On the point about leadership and tactics - they'll never build a party of more than 30, and will only recruit mercenaries/caravan guards (and maybe the odd converted, or released, prisoner). A stronger party can help them with bandits, but they'll basically always lose to a lord's party unless they can evade it. It's for this reason I'd prioritise speed (and, coincidentally, why the 'stronger troops' more expensive caravan isn't particularly useful).
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