re: acolyte; so, not a typical stormtrooper, more of a rogue ex-acolyte... that tells me so very, very little. Does he originate from
A) The military? If so, give him guardsmen cover.
B) The arbites? If so, give him arbitrators with combat shields, shockmauls and shotguns with special ammo.
C) A Tech-adept background? If so, give him combat servitors- possibly ones that float.
D) A Naval background? If so, give him naval support troops- like guardsmen, but with shotguns, possibly with special ammo.
E) The underworld? If so, was he reformed by...
1) An Ecclesiarchal figure? If so, give him arco-flaggelents.
2) A militaristic figure? If so, give him penal-legionnaires.
3) A Tech, Arbitrator or Naval figure? If so, as appropriate to the background.
The follow comes with an "Opinions stated here, your mileage may vary" stamp.
I mean, if we're talking about a lone gunman with a badass longcoat and a lascarbine... he should not be able to keep up with the Astartes in terms of combat. If he does, you obviate the point of the Astartes and nullify their superhumanity.
If you're worried about him keeping up, for one thing ditch the lascarbine and give him a heavier weapon- not Astartes grade, but...
If he's beefy enough to take Bulging Biceps, have him do so and let him take an Autocannon. Don't think of an autocannon in the terms you usually see it, but think of it as God's own sniper rifle, the 40k equivalent to a Barrett .50 rifle reaching out and snuffing out a life. At 3d10+8 pen 6, That's a carbine that'll make them feel the pain! Alternatively, if he'd rather fire support and pretend he's a midget devastator who favours rate of fire over damage, he could take a multilaser. This is how you re-enact these scenes from Predator, but with lasers.
If he isn't beefy but he's pretty agile, and you want him to be a nimbly bimbly little snot, have him take the 2-pistol talents. Then give him a quartet of pistols- 2x hotshot las pistols and, for his special weapon, 2x plasma pistols. Or 2x melta pistols. That's a lot of gunslinging hate.
I'd recommend starting off with a "standard" loadout, similar to what Astartes have, then giving him the chance to upgrade it mission to mission. If he's the Biceps man, start him off with a heavy stubber. If not, start with either a hotshot lasrifle or two hotshot laspistols.
I still prefer the horde rules, though.
Re: Lyn's squad rules; Rather than changing anything else, I'd recommend that each shot that hits must do at least one point of damage after all else- but cannot kill a Master or Elite level foe; similarly a guard who gets righteous fury will deal at least 2 damage (lucky shot to eye or a ***** in the natural armour, kind of like Smaug) after all else, and can kill a master level foe. Bear in mind that in the tabletop, any jerk with a lasgun can deal a wound on a Hive Tyrant. This allows the squad of buddies to contribute without stealing the show after an assault marine gets righteous fury with a power sword on a Tau Battlesuit but leaves one hitpoint left... unless one of the squadmates gets righteous fury as well.