Ugh! Lost a long reply...this new format sure seems buggy.
First of all, HUGS!
Your baby, adopted or not, is going through the most fussy time right now. 4-8 weeks can be ROUGH! All of our kids have had a period around then when they sometimes could not be soothed, when they seemed so uncomfortable and unhappy. I know it's really hard to go through, especially with two other kids to take care of. It is a pretty typical phase, though, and most kids pull through it in a few weeks. By the time you get to 9 or 10 weeks, your little girl should be feeling better. (Oh, and exhale the worry about being a high-needs baby for life...all of our kids that had the 5/6 week fussiness have turned out to be really calm, laid-back kids.)
What really helped us was giving them probiotics (Maxi Baby-Dophilus is the brand we used...it's $7 or so, and you can find it in stores or online). There was a study that came out a few years ago that showed babies with colic could get over the colic completely if they were given probiotics. Not all babies, but a very very large percentage of them. This was in babies that were 3 or 4 months old, but I've seen probiotics used with much younger babies, too. It makes total sense...colic is fussiness from gas or pain, and if you have an imbalance of gut flora you're going to have a lot of gas and discomfort. Probiotics help establish healthy gut flora...something all babies need in order to process their food well. Once they have that healthy gut flora, the gas isn't as bad, they're not as fussy, and they calm down.
The results were pretty immediate. Maybe 2, 3 days? We gave them probiotics (by putting the dry powder on our finger or the bottle nipple) about 3 times a day. If we stuck with it, we'd notice a few changes...1) the fussiness decreased 2) the gassiness and uncomfortable movements/squirming/arching decreased 3) their poop became more yellow and more thick (versus slimy/mucousy). We kept up probiotics for a couple of weeks, and by the time they get to 9 or 10 weeks we'd stop. We brought it back now and then if their poop seemed to get off-color again, but a little probiotic in those first several weeks did wonders.
It's worth a try. And even if it doesn't work, just remember that these few weeks are your hardest. Fissiness peaks soon, and by the time you get to 10 weeks you should start seeing this go away. PLEASE try probiotics, though. I've seen it work wonders, and I've heard from friends that it worked well for their kids, too.
Keep your chin up! I'm sure you're doing a great job!