Some others with scale in mind. JMO except where noted.
Slow and easy
Cryptocoryne pygmaea, lucens, willisii, walkeri.
C. wendtii variants, becketii, etc if you don't mind splitting it every few months (they get wide and big).
Anubias nana "petite"
Cladorpha / Moss balls
Monosolenium tenerum
Xmas, etc mosses. Xmas is still my favorite, but like all mosses it can grow relatively fast when established, and so may belong below.
~medium and higher difficulty. May require regular trimming to look to scale.
Rotala sp from Vietnam
R. rotundifolia
R. macrandra "narrow leaf" / R. "magenta"
Bacopa australis
Micranthemum umbrosum
Ludwigia brevipes
L. arcuata (I think, have not grown it)
Hydrocotyle sp
Ranunculus papulentus
Hygrophillia difformis / Wisteria
H. corymbosa "compact"
Elatine triandra
Blyxa japonica (can get too big and better as a midground)
Heteranthera zosterifolia / Stargrass (mid-back ground)
Alternanthera reineckii
Nymphea stellata / Dwarf lilies
Mayaca fluviatilis
Lobellia cardinalis "small form" (I think, see L. arcuata.)
Harder requirements, but still relatively slow growing
Ammania sp. "Bonsai"/Rotala Indica
Rotala sp. from Goias
most Eriocaulon sp.
Hemianthus micranthemoides (this is easier for other people)
Hard stems that can grow faster, but are cool
Tonina sp
R. wallichii
R. sp "Mini"/"Blue"/"Pearl" etc
R. pusilla
R. hippuris (I think.)
R. macrandra
Ludwigia sp from Pantanal (I think.)
