You've got Australian bows, fishinAZ, they get to 6"
40g breeder isn't a bad size, Unca. Nice and deep and fair length. Good choice for a smaller planted tank.
You sure you want to mix them? You can get a bigger school going with just praecox and some Boesemani aren't very nice to smaller bows. Just a thought.
If you are going to mix, you really should do a min of 2 girls to every boy, regardless of specie because they are not going to give a hoot an' hollar who they court. The praecox boys will court the boe girls and the boe boys the praecox girls. They don't care. Randy bunch
For a mix, I'd max at 6-7 with the majority being praecox. So, three boes and 4 praecox should work in a 36"x18" tank. The praecox go to about 3", but the Boesemani, Unca, can get to 5". The seven oldest I have were bought back in October -- four months ago -- and are over 3" easy. They were 2" when I brought them home.
Now, if you want a bright RED fish that will max under 3", don't mind a little higher price, and aren't in a huge hurry, then I would look at
pseudoincisus, the Millenium rainbowfish. You could get a school of 8-10 at least in that size of a tank. I have 7 babies in my 36g and they are all 1.5" and under. Tons of room still left in the tank. They start to color up at 2 months of age. Mine are just starting to show.
Another one in that range is
Melanotaenia parva, the Flame or Lake Kurumoi Rainbowfish. That one also maxes ~3" and very beautiful. That one is more available than the Millenium.
Just some thoughts and options
Lemme know if you are interested in that and I'll shoot you some info.
Roan