When you make squares (or anything) that will be seamed blocking to get them laying flat is a good idea and makes seaming easier. With acrylic a gentle wash after may be all you need.
In this case a blocking board would be handy to make squares a certain size. Acrylic won't stretch as much as wool, but maybe enough to make them a bit closer and easier to seam. Whether it'll be permanent I don't know, but when it's seamed it won't matter. I agree a row of crochet around the smaller ones would be a great idea, too.
I'm glad knitting looser is helping!!