I can certainly understand building something from the ground (er tectonic plates) up

Your second map looks better than the first. I would fiddle with the directions of the plates a bit though and have more plates in collision. For instance, you've got a massive mountain chain in the west of plate 9, yet plate 6 and plate 9 appear to both be heading "north" (or at least "up"). You are not going to get massive Himalaya type mountains (maybe I'm misinterpreting your mountain depictions, but it sure looks Himalaya like) unless you have two plates that are pushing/colliding against each other (which is what the Indian plate has been doing to the Asian plate for the last several million years).

Your triangles are fine as temporary markers for where you wish to place your islands. Not every plate collision will have islands, but there were a few areas that did not have islands that had me scratching my head a bit (such as the boundary at 9, 14, and 16 and the boundary between 2 and 17 which are pressing against each other). As for island size, you can make them any size you want, but I would have very large islands (New Zealand, Iceland, Japan sized) on or near the boundaries of 3 or more plates.

Keep working at it Laime, you have the fundamentals of a great map here.