I had a 3rd level thief, er specialist, in my game before he died precipitously recently, and we have a 1st level henchman. I found that they took a lot of search, tinkering and find traps at first level, giving them each a 50/50 on most die rolls. It was nice having thieves that didn't suck. The moment we got out of the dungeon, and stealth, languages and climb came into play, they saw the need to start diversifying; I'm not worried about overloading in a regular game.
In a megadungeon like Nightwick Abbey, I guess there's the risk they overspecialize unless you explicitly warn them you'll have situations requiring the skills of well-rounded specialists.
The companion set question is tough though... I'd still keep the projected campaign arc as 1-10 or 1-12 levels like B/X or WFRP, and just integrate the companion rules earlier; allow dominions earlier, use the war machine. I really hated when Mentzer came out and thief skills, that were scaled for lower levels, suddenly had to stretch to level 36 and B/X level thieves took a big hit.