For a 30g I would recommend botia striata, or botia kubotai. Both stay a bit smaller and are just a bit more peaceful than yoyos. But, that being said, I have never had a problem with yoyos being aggressive either. They do need to be kept in groups, I would say 3-4 is the absolute minimum, but they much prefer 5+. I would not add other bottom feeders, because I think loachs tend to outcompete them for food. If he just has one large yoyo, I would suggest adding maybe 4-5 more smaller loaches (Striata or Kubotai). They don't need others of their same species to school, just other botia loaches. The yoyo will be much happier, and I bet he'll leave the other fish alone more sine he will have company of his own kind.
Also, loaches are usually fine with plants, but they do better with rooted plants like crypts, vals, swords, or well rooted stem plants, and mosses or floating plants. They may pull up stuff if it doesn't have good roots, but it's not a huge problem.
Re: tanks size, 30g may not be ideal for a yoyo, but I don't think it is terrible, and I think it is perfectly fine for zebra loaches (botia striata). I kept 5 of them in a 20g for years. If you get the yoyo some smaller botias for company I'd say it would be fine, as long as he stays on top of the water quality.