Yeah, acei and yellow labs are 2 of the "less aggressive" mbuna species. The rusty (Iodotropheus sprengerae) and Cynotilapia afra can be on that list too.
Yellow Labs are probably the least aggressive Mbuna in general. They do require a 55 gallon long tank at least. Yellow-Tail Aceis get a little bigger, and should properly be in 75 at least, but would really get by in a 55. Aceis are a little more aggressive than Labs but it's hard to find any other Mbuna that are as peaceful as Labs.