It looks like you have it sorted out. Don't give up on fine tuning, I am sure any trouble is caused by minor issues of maintanance. A 15 is as close to bullit proof as it gets, it takes any thread within reasonable quality (and a 15x1 needle can handle). Cotton, polyester, and blends should be fine; linen and silk if required. I can let you know Guttermann poly is the least messy, much less dust. Mercerized cotton is nice too.
It looks like your 15-90 have the needle go in with the flat side towards the left, and threads right to left. It should be a very well behaved, capable and easy to use machine. Another basic thing important to get right is to match thread and needle size. For top stitch thread commonly used on denim you max out on neede size right away, at least #18, even #19 or #20 of you can find it.
Here is a manual for the 15-90. It should shine up very well with liquid car polish (resin type), and the metal bars usually shine up like new with the right polish, like Quick-Glo