Add 18 Plains objects. This makes Raticate, Chansey, and Seedot take slots 0, 1, and 2 respectively. Skiploom is bumped up to slot 3 and Geodude/Clefairy are slot 4, Riolu is 5.
However, I forgot if there was a limit to the number of blocks you can place in an area... so that may not work if there is. If there's a limit (iirc, there was a limit of 30, but Idk if that limit was for all blocks in general, or just per one type of block) I think the most you can get Riolu up to is slot 3 :/