Olimar has a meh tech, Squirtle and Toon Link have some of the worst techs I've seen in any smash game. Downthrow works on them for days. Just read the direction and follow and grab, JC grab may be faster/easier yes? If they just lay there I usually jab reset into dtilt then depending on DI either uair or fair and try to land another fair to get them off stage. Downthrow is stupid on platforms. You can dthrow and then cover most all options except getup attack with Dsmash.
I've been getting a lot of work out of fair lately. I found that I told myself it was so bad I just stopped using it as much. It is pretty good, but still slow imo. I haven't faced too many people, but I have found that bair to nair works on a lot of characters, and bair to fair works on even more. Even if it doesn't work guaranteed via hitstun, it still puts them disadvantaged above you or next to you for you to bait and punish.
And yes uthrow into whatever works on Roy. You can land SideB as well, but I usually go for nair since it covers the most area. Be careful of platforms they can tech on, and landing nair so that it sends them accross the stage instead of into the blast zone.