I always hope testers will use whatever yarn and hoook combo will allow them to meet gauge and be a successful version of the project (drapery yarn isn’t for every project, etc). I try to give a thorough indication of what qualities would be good to have in the yarn, and the weight of course. Sometimes I have a few suggested yarns I think would work well, but my testers can choose whatever. There are advantages to a tester using an inappropriate for the project yarn too. You never know what a customer will choose, so when I’ve seen the results, I may have some extra advice like “best suited for acrylic”, or “yarn must have good drape kr item will be very stiff” or whatever is appropriate to mention. Honestly, I try to give that kind of advice to the testers too, but some people will choose what they want no matter what and then there’s proof!
Whatever hook is co for table and meets gauge is always the right hook. I try to phrase it as recommended hook, and include “or hook to meet gauge”. I find even for myself, my own gauge varies between. my two sets of hooks.
As far as alterations go, if a part isn’t working out and you see the issue, a wrong stitch count in a direction or something like that, that’s where I love “it didn’t work for me as written. X worked instead, is this what you meant? “ before continuing last that point. There’s a chance I meant something entirely different, changes to the style itself are not welcome to me. If someone wants to buy my pattern and use it as a foundation for their idea, cool. Especially when they still give me credit for designing the base pattern. But that’s not the purpose of a test, it’s to make sure the pattern works as written, so if you don’t love it in the designers sample photos, maybe it’s not for you until it goes on sale and you can buy it and make it suit yourself.
That being said, sometimes, I may suggest a couple variations, or ways to change part of it up a little. In these cases, I welcome creativity, as long as it stays within the bounds of the pattern (still following the instructions on the page). I had one hat pattern written for dk weight. I also listed advice for working in any yarn weight by altering stitch counts and giving measurements to reach to find the counts. I was super happy to have a tester decide to use another weight. And had at least 2 more testers who loved the pattern make at least one more hat with weights other than dk after finishing gushier first, all before the test deadline. This is ok by me because they were testing another aspect of the pattern. I’m also good with people adding or removing length to body or sleeves to fit. Or testing a C2C graph in another weight to be used as a wall hanging or pillow cover instead of a blanket as long as I know ahead that’s the intention in case I really need it done “ by the book” (I’ve offered this idea to testers but haven’t had anyone take me up on it yet). Basically, things that show the versatility of the pattern without altering the pattern and I like to know first.
Hope this is useful, a little wordy, it I figure it’s easier to understand why it usually works he way it does if you know where the other side is coming from.