It is fine to have several different chapters in one space marine army, as long as they are all from the same codex. If they are well-painted, and look to be the same army, they will do well. For example, each squad have an army badge in addition to their chapter colors and iconagraphy. Also, ensure all the squads have similar basing that ties the army together.
Okay, that's good to know. I'm not at the basing stage yet but will get there soon enough. Thankfully I have enough Crimson Fist decals to go around. Furthermore, I have 20 Ultramarines with the shoulder pads.
In regards to iconography, is the arrow on the right pad necessary?