By default, when you use Richard's ability, you are limited to cards legal in the format you're playing
Ultron is good alongside any artifact that has enters or leaves triggers, but Solemn Simulacrum is the gold standard when it comes to that kind of card
It just so happened that both sets were being printed in the same facility
Generally speaking, there should always be more cost 1 cards than cost 2 cards, more cost 2 cards than cost 3 cards, and so on and so forth
The only time I wouldnt include this is if I was playing a one-color deck
Because Breena is so efficient at loading up on +1/+1 counters I've leaned into it with a strong counter subtheme: we use those counters to draw cards off Dusk Legion Duelist, ramp with Scholar of New Horizons, have board wipe insurance with Felisa, Fang of Silverquill, and turn our team into lethal angels with Sigarda's Summons