Generally, after two or three washes, you don’t need to separate any more. You really only get bleeding with new fabrics, and even then only with cheap fabrics. As an example, if you order Amazon’s cheapest towels, not only wash them separately, wash them before you use them unless you want the color on you. But even those eventually stop bleeding.

If you’re going to sort, the way I was taught is to separate colors from whites, with blacks being a separate load as well. The idea is that the bleed from any of the colors isn’t going to be enough to matter since there’s already dye present, which means there’s less ability to take on new dye. That can fail, and you end up with the color of things changed. So you still always test any new fabrics before throwing them in with other colors. But, generally speaking, if you don’t have the time, don’t have enough of that color to make a load, or just don’t care about color change, the risk of any color change being huge is low nowadays.

The only reason to do blacks on their own is that it mutes other colors when it bleeds.

If you’re really paranoid about it, either do each color on their own, or make batches where the colors bleeding wont be as significant. Like, blues and greens, or greens and yellows. Any bleed that does happen like that won’t be as noticeable, if it is at all.

But I never saw in all my years washing other people’s clothes anyone that had enough of every color to make multiple loads without waiting way too long between washings. Maybe in a really big family you’d pile up enough mixed colors in a week or two to make full loads of multiple single colors, or even two colors.

You’re also usually okay washing bed sheets and clothes together, though you can pull out stuff with zippers and metal buttons if you take want to maximize life spans of the rest. Being real though, making a load of those together is just going to shift which items get the teeny, tiny extra bit of wear from the fasteners. It might be worth it with heavy denim, but I wouldn’t and don’t bother personally.

Now, as far as types of fabric, you run into some issues. Good wool might need its own special care, but you’d want to refer to the label to determine that. Silk is hand wash only unless you like ruining silk. Cotton, linen, polyester, rayon, and nylon can all be washed with any other fabrics, no issues. At worst, you might run into a little extra pilling with the natural fabrics being washed together, but I’ve sever seen it actually happen more than what the fabrics do on their own just from being washed.

You specified merino wool. Afaik, you should machine wash as the default. But wash in cold only, and go with the bare minimum of detergent. Make sure you pull it out as soon as possible to reduce wrinkling, and hang to dry. Yeah, in theory your can machine dry merino, but if you’re asking this at all, you want to maximize the life and appearance of your clothes, so hang to dry.

Hand washing works as well as you’re willing to put in the work on, but wool tends to hold on to oils and sweat residue more than most fabrics, so you have to really work at it compared to something like cotton that gives up oils as easily as any natural fabric will.

If you’re having a problem with lint on your laundry, do your towels and other terry fabrics on their own. Most of the time, that’s where excessive lint is coming from. But if you aren’t having that issue, don’t worry about it.

A lot of separating of types of fabric goods, like sheets from clothes, isn’t about what they’ll do to each other or needing different types of care, it’s about convenience. Folding and storing sheets all at once is easier than dealing with a mixed load, as an example. You do benefit in being able to run things like towels hotter than you’d want with most clothing, but it’s a marginal benefit imo.

My laundry isn’t sorted much at all unless I get new stuff. Towels and washcloths and the like stay in their own loads. Clothes together, with only a few extra heavy fabrics on their own (like my canvas outdoors stuff and gis). Bed linens tend to be a load of their own since there’s two beds worth of those at a time, there’s just not room for more. But, when that’s not the case, I tend to throw them in with towels.

And, as always, check your labels. While it isn’t a guarantee, the recommended care on them really is the best pick. You’ll run into some chinese fabrics where it’s just generic instructions, but those tend to default to the least wear options anyway, so it won’t hurt anything, it just isn’t ideal. The companies using outsourced labor still specify the label contents, so it’s only when it isn’t a brand at all that you see the generic labels.

I also tend to recommend that you avoid fabric softener. They really don’t do anything useful, they cost extra, and a lot of people are sensitive to them and don’t realize it. If you’re getting itchy a lot, and you’re using them, try a few weeks without. Same with rashes where the fabric is extra close to skin.