Early on I used Dexter for a while, but eventually he didn't seem to have a niche. I hate having to lose a shot moving him forward, but I hate getting second-rate damage rolls even more. That .45 can't hit much harder than a rifle against Duraplate so he's not much good against top-tier opponents. The extra crits sound like fun though, I dunno. We'll see where I can fit him in.
That's my plan with Doyle too, let him keep his trusty rifle most of the time but give him a plasma/coil gun for the tough ones.
Bud will get his aim up, it's inevitable in my view.
Well I don't think either Dexter or Bud ever qualify as top tier no matter what you do. They're both solid mid tiers though. 75-100 health just doesn't cut it for the toughest fights, but if you're going to run with only 1 top tier for awhile that means you need 3 low to mid tiers to fill in the rest of your squad. Doyle is pretty much a lock for 1 of those spots IMO when you combine his already high aim with his 150 health. I wouldn't want to pay for plasma ammo on him all the time but I could definitely see tossing him a plasma gun for a tier 5-6 bounty and then swapping it back out again for regular fights. I do the same with my main. He uses a rifle most of the time and swaps in a plasma for the tough fights.
That leaves 2 more spots. Bud is a good mid term option since he can eventually use a plasma for free, but he starts with such low aim that IMO training him several hours a day til he hits something like 80-90 aim is mandatory if you want to use him longer than about a month. That's a lot of time that could have been spent doing something else but it does give you some serious power on the cheap if you do it. The problem is he still only has 75 health. So a plasma can 1 shot him (or close enough to it that pretty much any follow up hit will take him down). 2 shots with a coil gun or plasma can also take him down and it's tough to guarantee that the enemy never gets more than 1 shot in a tough fight.
Dexter has an extra 25 health compared to most of the other mercs, decent aim, bonus crit damage (more than a rifle), and pays for his own ammo. So to me he's a lock for that third mid tier slot unless you plan on running with only 1 top tier for long enough to train a 2nd low tier up above 90 aim and pay for their ammo. In that case I think Aiko probably is the best bet, and I do pick her up early on for her slightly better aim compared to Loren.
The other problem I have with Bud is that it takes me awhile to get to the point where I can really train him significantly. The first week I'm spending all of my time training stamina, upgrading my garden/bedroom, killing stuff, questing, and getting massages. The 2nd week I can squeeze in a little time for Bud's attention whore thing but little to nothing for actual training. It's only when I hit 60 stamina that I finally feel like I have enough stamina to actually do more than 1 or 2 training sessions a day with bud and still do anything else other than get massages all day and all night.
That eventually gives you an option for 4 plasma guns (or 3 plus Ayden's crazy coil gun depending on your top tier) for the toughest fights and then Dexter just putting in somewhat solid damage with his pistol plus having a little better durability most of the other mercs. You could bump that up to 5 if you replaced Dexter with someone else you were going to pay for ammo on, but if you really need 5 plasmas for a fight IMO it's time to just grab at least 3 of the top tiers anyway. You could get away with running with 3 top tiers plus Bud most of the time and just swapping in Doyle for the absolute toughest fights, but by then you probably have the passive income to support all 4 top tiers anyway, and would no longer have to "spend time" with Bud so that would free up some more time to do whatever else you want to focus on.